Parameter derivation in cross component mode

ABSTRACT

A method for visual media processing, including performing a conversion between a current chroma video block of visual media data and a bitstream representation of the current chroma video block, wherein, during the conversion, a chroma residual of the current chroma video block is scaled based on a scaling coefficient, wherein the scaling coefficient is derived at least based on luma samples located in predefined positions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/CN2020/085674, filed on Apr. 20, 2020, which claims the priority to and benefit of International Patent Application No. PCT/CN2019/083320, filed on Apr. 18, 2019. All the aforementioned patent applications are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

This patent document relates to video coding and decoding techniques, devices and systems.

BACKGROUND

In spite of the advances in video compression, digital video still accounts for the largest bandwidth use on the internet and other digital communication networks. As the number of connected user devices capable of receiving and displaying video increases, it is expected that the bandwidth demand for digital video usage will continue to grow.

SUMMARY

Devices, systems and methods related to digital video coding/decoding, and specifically, simplified linear model derivations for the cross-component linear model (CCLM) prediction mode in video coding/decoding are described. The described methods may be applied to both the existing video coding standards (e.g., High Efficiency Video Coding (HEVC)) and future video coding standards (e.g., Versatile Video Coding (VVC)) or codecs.

In one representative aspect, a method for visual media processing is disclosed. The method includes performing a conversion between a current chroma video block of visual media data and a bitstream representation of the current chroma video block, wherein, during the conversion, a chroma residual of the current chroma video block is scaled based on a scaling coefficient, wherein the scaling coefficient is derived at least based on luma samples located in predefined positions.

In one representative aspect, a method for visual media processing is disclosed. The method includes performing a conversion between a current video block of visual media data and a bitstream representation of the current video block, wherein, during the conversion, a second set of color component values of the current video block are derived, using a cross-component linear model (CCLM) and/or a luma mapping with chroma scaling (LMCS) mode processing step, from a first set of color component values of the visual media data.

In another representative aspect, a method for visual media processing is disclosed. The method includes performing a conversion between a current video block of visual media data and a bitstream representation of the current video block, wherein, during the conversion, one or more reconstruction samples associated with a current frame of the visual media data are used to derive a chroma residual scaling factor in a luma mapping with chroma scaling (LMCS) mode processing step.

In another representative aspect, a method for visual media processing is disclosed. The method includes performing a conversion between a current video block of visual media data and a bitstream representation of the current video block, wherein, during the conversion, one or more luma prediction samples or luma reconstruction samples in a current frame other than a reference frame and are used to derive a chroma residual scaling factor in a luma mapping with chroma scaling (LMCS) mode processing step.

In another representative aspect, a method for visual media processing is disclosed. The method includes checking, during a conversion between a current chroma video block and a bitstream representation of the current chroma video block, availability of one or more neighboring luma blocks of a corresponding luma block which covers a top-left sample of a collocated luma block; determining, based on the availability of one or more neighboring luma blocks, whether to retrieve neighboring luma samples of the corresponding luma block; deriving, based on the determining, a scaling coefficient; scaling, based on the scaling coefficient, a chroma residual of the current chroma video block to generate a scaled chroma residual; and performing the conversion based on the scaled chroma residual.

In another representative aspect, a method for visual media processing is disclosed. The method includes deriving, during a conversion between a current video block of visual media data and a bitstream representation of the current video block, a second set of color component values of the current video block from a first set of color component values of the visual media data using a model associated with a processing step, wherein the first set of color component values are neighboring samples of a corresponding luma block which covers a top-left sample of a collocated luma block.

In another representative aspect, a method for visual media processing is disclosed. The method includes during a conversion between a current chroma video block of visual media data and a bitstream representation of the current chroma video block, making a determination of selectively enabling or disabling application of a cross-component linear model (CCLM) and/or a chroma residual scaling (CRS) on the current chroma video block, based at least in part on one or more conditions associated with a collocated luma block of the current chroma video block.

In another representative aspect, a method for visual media processing is disclosed. The method includes selectively enabling or disabling application of a luma dependent chroma residual scaling (CRS) on chroma components of a current video block of visual media data for encoding the current video block in a video region of a visual media data into a bitstream representation of the visual media data; and making a determination of including or excluding a field in the bitstream representation of the visual media data, wherein the field is indicative of the selectively enabling or disabling and, if included, is signaled, other than at a first syntax level associated with the current video block.

In another representative aspect, a method for visual media processing is disclosed. The method includes parsing a field in a bitstream representation of visual media data, wherein the field is included in a level other than at a first syntax level associated with a current video block; and selectively enabling or disabling, based on the field, application of a luma dependent chroma residual scaling (CRS) on chroma components of the current video block of visual media data for generating a decoded video region from the bitstream representation.

In another representative aspect, a method for visual media processing is disclosed. The method includes selectively enabling or disabling application of a cross-component linear model (CCLM) on a current video block of visual media data for encoding the current video block into a bitstream representation of the visual media data; and making a determination of including or excluding a field in a bitstream representation of the visual media data, wherein the field is indicative of the selectively enabling or disabling and, if included, is signaled, other than at a first syntax level associated with the current video block.

In another representative aspect, a method for visual media processing is disclosed. The method includes parsing a field in a bitstream representation of visual media data, wherein the field is included a level other than at a first syntax level associated with a current video block; and selectively enabling or disabling, based on the field, application of a cross-component linear model (CCLM) on the current video block of visual media data for generating a decoded video region from the bitstream representation.

In yet another example aspect, a video encoder or decoder apparatus comprising a processor configured to implement an above described method is disclosed.

In another example aspect, a computer readable program medium is disclosed. The medium stores code that embodies processor executable instructions for implementing one of the disclosed methods.

In yet another representative aspect, the above-described method is embodied in the form of processor-executable code and stored in a computer-readable program medium.

In yet another representative aspect, a device that is configured or operable to perform the above-described method is disclosed. The device may include a processor that is programmed to implement this method.

The above and other aspects and features of the disclosed technology are described in greater detail in the drawings, the description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of angular intra prediction modes in HEVC.

FIG. 2 shows an example of directional modes not in HEVC.

FIG. 3 shows an example in connection with the CCLM mode.

FIG. 4 shows an example of luma mapping with chroma scaling architecture.

FIG. 5 shows an example of luma block and chroma block in different color formats.

FIG. 6 shows an example of luma block and chroma block in same color formats.

FIG. 7 shows an example of collocated luma block covering multiple formats.

FIG. 8 shows an example of luma block within a larger luma block.

FIG. 9 shows an example of luma block within a larger luma block and within a bounding box.

FIG. 10 is a block diagram of an example of a hardware platform for implementing a visual media decoding or a visual media encoding technique described in the present document.

FIG. 11 shows a flowchart of an example method for linear model derivations for cross-component prediction in accordance with the disclosed technology.

FIG. 12 is a block diagram of an example video processing system in which disclosed techniques may be implemented.

FIG. 13 shows a flowchart of an example method for visual media processing.

FIG. 14 shows a flowchart of an example method for visual media processing.

FIG. 15 shows a flowchart of an example method for visual media processing.

FIG. 16 shows a flowchart of an example method for visual media processing.

FIG. 17 shows a flowchart of an example method for visual media processing.

FIG. 18 shows a flowchart of an example method for visual media processing.

FIG. 19 shows a flowchart of an example method for visual media processing.

FIG. 20 shows a flowchart of an example method for visual media processing.

FIG. 21 shows a flowchart of an example method for visual media processing.

FIG. 22 shows a flowchart of an example method for visual media processing.

FIG. 23 shows a flowchart of an example method for visual media processing.

DETAILED DESCRIPTION 2.1 A Brief Review on HEVC 2.1.1 Intra Prediction in HEVC/H.265

Intra prediction involves producing samples for a given TB (transform block) using samples previously reconstructed in the considered colour channel. The intra prediction mode is separately signalled for the luma and chroma channels, with the chroma channel intra prediction mode optionally dependent on the luma channel intra prediction mode via the ‘DM_CHROMA’ mode. Although the intra prediction mode is signalled at the PB (prediction block) level, the intra prediction process is applied at the TB level, in accordance with the residual quad-tree hierarchy for the CU, thereby allowing the coding of one TB to have an effect on the coding of the next TB within the CU, and therefore reducing the distance to the samples used as reference values.

HEVC includes 35 intra prediction modes a DC mode, a planar mode and 33 directional, or ‘angular’ intra prediction modes. The 33 angular intra prediction modes are illustrated in FIG. 1 .

For PBs associated with chroma colour channels, the intra prediction mode is specified as either planar, DC, horizontal, vertical, ‘DM_CHROMA’ mode or sometimes diagonal mode ‘34’.

Note for chroma formats 4:2:2 and 4:2:0, the chroma PB may overlap two or four (respectively) luma PBs; in this case the luma direction for DM_CHROMA is taken from the top left of these luma PBs.

The DM_CHROMA mode indicates that the intra prediction mode of the luma colour channel PB is applied to the chroma colour channel PBs. Since this is relatively common, the most-probable-mode coding scheme of the intra_chroma_pred_mode is biased in favor of this mode being selected.

2.2 Versatile Video Coding (VVC) Algorithm Description 2.2.1 VVC Coding Architecture

To explore the future video coding technologies beyond HEVC, Joint Video Exploration Team (JVET) was founded by VCEG and MPEG jointly in 2015. The JVET meeting is concurrently held once every quarter, and the new coding standard is targeting at 50% bitrate reduction as compared to HEVC. The new video coding standard was officially named as Versatile Video Coding (VVC) in the April 2018 JVET meeting, and the first version of VVC test model (VTM) was released at that time. As there are continuous effort contributing to VVC standardization, new coding techniques are being adopted to the VVC standard in every JVET meeting. The VVC working draft and test model VTM are then updated after every meeting. The VVC project is now aiming for technical completion (FDIS) at the July 2020 meeting.

As in most preceding standards, VVC has a block-based hybrid coding architecture, combining inter-picture and intra-picture prediction and transform coding with entropy coding. The picture partitioning structure divides the input video into blocks called coding tree units (CTUs). A CTU is split using a quadtree with nested multi-type tree structure into coding units (CUs), with a leaf coding unit (CU) defining a region sharing the same prediction mode (e.g. intra or inter). In this document, the term ‘unit’ defines a region of an image covering all colour components; the term ‘block’ is used to define a region covering a particular colour component (e.g. luma), and may differ in spatial location when considering the chroma sampling format such as 4:2:0.

2.2.2 Dual/Separate Tree Partition in VVC

Luma component and chroma component can have separate partition trees for I slices. Separate tree partitioning is under 64×64 block level instead of CTU level. In VTM software, there is an SPS flag to control the dual-tree on and off.

2.2.3 Intra Prediction in VVC 2.2.3.1 67 Intra Prediction Modes

To capture the arbitrary edge directions presented in natural video, the number of directional intra modes in VTM4 is extended from 33, as used in HEVC, to 65. The new directional modes not in HEVC are depicted as red dotted arrows in FIG. 2 , and the planar and DC modes remain the same. These denser directional intra prediction modes apply for all block sizes and for both luma and chroma intra predictions.

2.2.3.2 Cross-Component Linear Model Prediction (CCLM)

To reduce the cross-component redundancy, a cross-component linear model (CCLM) prediction mode is used in the VTM4, for which the chroma samples are predicted based on the reconstructed luma samples of the same CU by using a linear model as follows: pred_(C)(i,j)=α·rec_(L)′(i,j)+β where pred_(C)(i,j) represents the predicted chroma samples in a CU and rec_(L)(i,j) represents the downsampled reconstructed luma samples of the same CU. Linear model parameter α and β are derived from the relation between luma values and chroma values from two samples, which are luma sample with minimum sample value and with maximum sample inside the set of downsampled neighboring luma samples, and their corresponding chroma samples. The linear model parameters α and β are obtained according to the following equations.

$\alpha = \frac{Y_{a} - Y_{b}}{X_{a} - X_{b}}$ β = Y_(b) − α ⋅ X_(b)

Where Y_(a) and X_(a) represent luma value and chroma value of the luma sample with maximum luma sample value. And X_(b) and Y_(b) represent luma value and chroma value of the luma sample with minimum luma sample, respectively. FIG. 3 shows an example of the location of the left and above samples and the sample of the current block involved in the CCLM mode.

The division operation to calculate parameter α is implemented with a look-up table. To reduce the memory required for storing the table, the diff value (difference between maximum and minimum values) and the parameter α are expressed by an exponential notation. For example, diff is approximated with a 4-bit significant part and an exponent. Consequently, the table for 1/diff is reduced into 16 elements for 16 values of the significand as follows: DivTable[ ]1={0,7,6,5,5,4,4,3,3,2,2,1,1,1,1,0}

This would have a benefit of both reducing the complexity of the calculation as well as the memory size required for storing the needed tables

Besides the above template and left template can be used to calculate the linear model coefficients together, they also can be used alternatively in the other 2 LM modes, called LM_A, and LM_L modes.

In LM_A mode, only the above template are used to calculate the linear model coefficients. To get more samples, the above template are extended to (W+H). In LM_L mode, only left template are used to calculate the linear model coefficients. To get more samples, the left template are extended to (H+W).

For a non-square block, the above template are extended to W+W, the left template are extended to H+H.

To match the chroma sample locations for 4:2:0 video sequences, two types of downsampling filter are applied to luma samples to achieve 2 to 1 downsampling ratio in both horizontal and vertical directions. The selection of downsampling filter is specified by a SPS level flag. The two downsampling filters are as follows, which are corresponding to “type-0” and “type-2” content, respectively.

$\mspace{79mu}{{{rec}_{L}^{\prime}\left( {i,j} \right)} = \ {\begin{bmatrix} {{re{c_{L}\left( {{{2i} - 1},{{2j} - 1}} \right)}} + {2 \cdot {{rec}_{L}\left( {{{2i} - 1},{{2j} - 1}} \right)}} +} \\ {{{rec}_{L}\left( {{{2i} + 1},{{2j} - 1}} \right)} + {{rec}_{L}\left( {{{2i} - 1},{2j}} \right)} +} \\ {{2 \cdot {{rec}_{L}\left( {{2i},{2j}} \right)}} + {{rec}_{L}\left( {{{2i} + 1},{2j}} \right)} + 4} \end{bmatrix} ⪢ 3}}$ ${{rec}_{L}^{\prime}\left( {i,j} \right)} = {\begin{bmatrix} {{{rec}_{L}\left( {{2i},{{2j} - 1}} \right)} + {{rec}_{L}\left( {{{2i} - 1},{2j}} \right)} + {4 \cdot {{rec}_{L}\left( {{2i},{2j}} \right)}} +} \\ {{{rec}_{L}\left( {{{2i} + 1},{2j}} \right)} + {{rec}_{L}\left( {{2i},{{2j} + 1}} \right)} + 4} \end{bmatrix} ⪢ 3}$

Note that only one luma line (general line buffer in intra prediction) is used to make the downsampled luma samples when the upper reference line is at the CTU boundary.

This parameter computation is performed as part of the decoding process, and is not just as an encoder search operation. As a result, no syntax is used to convey the α and β values to the decoder.

For chroma intra mode coding, a total of 8 intra modes are allowed for chroma intra mode coding. Those modes include five traditional intra modes and three cross-component linear model modes (CCLM, LM_A, and LM_L). Chroma mode coding directly depends on the intra prediction mode of the corresponding luma block. Since separate block partitioning structure for luma and chroma components is enabled in I slices, one chroma block may correspond to multiple luma blocks. Therefore, for Chroma DM mode, the intra prediction mode of the corresponding luma block covering the center position of the current chroma block is directly inherited.

2.2.3.2.1 Corresponding Modified Working Draft (JVET-N0271)

The following spec is based on the modified working draft of JVET-M1001 and the adoption in JVET-N0271. The modification of the adopted JVET-N0220 is shown in bold and underlining.

Syntax table

Semantics sps_cclm_enabled_flag equal to 0 specifies that the cross-component linear model intra prediction from luma component to chroma component is disabled. sps_cclm_enabled_flag equal to 1 specifies that the cross-component linear model intra prediction from luma component to chroma componenent is enabled. Decoding Process In 8.4.4.2.8 Specification of INTRA_LT_CCLM, INTRA_L_CCLM and INTRA_T_CCLM intra prediction mode

Inputs to this process are:

-   -   the intra prediction mode predModeIntra,     -   a sample location (xTbC,yTbC) of the top-left sample of the         current transform block relative to the top-left sample of the         current picture,     -   a variable nTbW specifying the transform block width,     -   a variable nTbH specifying the transform block height,     -   chroma neighbouring samples p[x][y], with x=1, y=0 . . .         2*nTbH−1 and x=0 . . . 2*nTbW−1, y=−1.

Output of this process are predicted samples predSamples[x][y], with x=0 . . . nTbW−1, y=0 . . . nTbH−1.

The current luma location (xTbY, yTbY) is derived as follows: (xTbY,yTbY)=(xTbC<<1,yTbC<<1)  (8-156)

The variables availL, availT and availTL are derived as follows:

-   -   The availability of left neighbouring samples derivation process         for a block as specified in clause 6.4.X [Ed. (BB): Neighbouring         blocks availability checking process tbd] is invoked with the         current chroma location (xCurr, yCurr) set equal to (xTbC,yTbC)         and the neighbouring chroma location (xTbC−1,yTbC) as inputs,         and the output is assigned to availL.     -   The availability of top neighbouring samples derivation process         for a block as specified in clause 6.4.X [Ed. (BB): Neighbouring         blocks availability checking process tbd] is invoked with the         current chroma location (xCurr, yCurr) set equal to (xTbC,yTbC)         and the neighbouring chroma location (xTbC, yTbC−1) as inputs,         and the output is assigned to availT.     -   The availability of top-left neighbouring samples derivation         process for a block as specified in clause 6.4.X [Ed. (BB):         Neighbouring blocks availability checking process tbd] is         invoked with the current chroma location (xCurr, yCurr) set         equal to (xTbC, yTbC) and the neighbouring chroma location         (xTbC−1, yTbC−1) as inputs, and the output is assigned to         availTL.     -   The number of available top-right neighbouring chroma samples         numTopRight is derived as follows:         -   The variable numTopRight is set equal to 0 and availTR is             set equal to TRUE.         -   When predModeIntra is equal to INTRA_T_CCLM, the following             applies for x=nTbW . . . 2*nTbW−1 until availTR is equal to             FALSE or x is equal to 2*nTbW−1:             -   The availability derivation process for a block as                 specified in clause 6.4.X [Ed. (BB): Neighbouring blocks                 availability checking process tbd] is invoked with the                 current chroma location (xCurr, yCurr) set equal to                 (xTbC, yTbC) and the neighbouring chroma location                 (xTbC+x, yTbC−1) as inputs, and the output is assigned                 to availableTR             -   When availableTR is equal to TRUE, numTopRight is                 incremented by one.     -   The number of available left-below neighbouring chroma samples         numLeftBelow is derived as follows:         -   The variable numLeftBelow is set equal to 0 and availLB is             set equal to TRUE.         -   When predModeIntra is equal to INTRA_L_CCLM, the following             applies for y=nTbH . . . 2*nTbH−1 until availLB is equal to             FALSE or y is equal to 2*nTbH−1:             -   The availability derivation process for a block as                 specified in clause 6.4.X [Ed. (BB): Neighbouring blocks                 availability checking process tbd] is invoked with the                 current chroma location (xCurr, yCurr) set equal to                 (xTbC, yTbC) and the neighbouring chroma location                 (xTbC−1, yTbC+y) as inputs, and the output is assigned                 to availableLB             -   When availableLB is equal to TRUE, numLeftBelow is                 incremented by one.

The number of available neighbouring chroma samples on the top and top-right numTopSamp and the number of available neighbouring chroma samples on the left and left-below nLeftSamp are derived as follows:

-   -   If predModeIntra is equal to INTRA_LT_CCLM, the following         applies:         numSampT=availT?nTbW: 0  (8-157)         numSampL=availL?nTbH: 0  (8-158)     -   Otherwise, the following applies:         numSampT=(availT&&         predModeIntra==INTRA_T_CCLM)?(nTbW+Min(numTopRight,nTbH)):         0  (8-159)         numSampL=(availL&&         predModeIntra==INTRA_L_CCLM)?(nTbH+Min(numLeftBelow,nTbW)):0  (8-160)

The variable bCTUboundary is derived as follows: bCTUboundary=(yTbC &(1<<(CtbLog2SizeY−1)−1)==0)?TRUE:FALSE  (8-161)

The variable cntN and array pickPosN[ ] with N being replaced by L and T, are derived as follows:

-   -   The variable numIs4N is set equal to ((availT && availL &&         predModeIntra==INTRA_LT_CCLM)? 0:1).     -   The variable startPosN is set equal to numSampN>>(2+numIs4N).     -   The variable pickStepN is set equal to Max(1,         numSampN>>(1+numIs4N)).     -   If availN is equal to TRUE and predModeIntra is equal to         INTRA_LT_CCLM or INTRA_N_CCLM, cntN is set equal to         Min(numSampN, (1+numIs4N)<<1), and pickPosN[pos] is set equal to         (startPosN+pos*pickStepN), with pos=0 . . . (cntN−1).     -   Otherwise, cntN is set equal to 0.

The prediction samples predSamples[x][y] with x=0 . . . nTbW−1, y=0 . . . nTbH−1 are derived as follows:

-   -   If both numSampL and numSampT are equal to 0, the following         applies:         predSamples[x][y]=1<<(BitDepth_(C)−1)  (8-162)     -   Otherwise, the following ordered steps apply:         -   1. The collocated luma samples pY[x][y] with x=0 . . .             nTbW*2−1, y=0 . . . nTbH*2−1 are set equal to the             reconstructed luma samples prior to the deblocking filter             process at the locations (xTbY+x, yTbY+y).         -   2. The neighbouring luma samples pY[x][y] are derived as             follows:             -   When numSampL is greater than 0, the neighbouring left                 luma samples pY[x][y] with x=1 . . . −3, y=0 . . .                 2*numSampL−1, are set equal to the reconstructed luma                 samples prior to the deblocking filter process at the                 locations (xTbY+x, yTbY+y).             -   When numSampT is greater than 0, the neighbouring top                 luma samples pY[x][y] with x=0 . . . 2*numSampT−1, y=1,                 −2, are set equal to the reconstructed luma samples                 prior to the deblocking filter process at the locations                 (xTbY+x, yTbY+y).             -   When availTL is equal to TRUE, the neighbouring top-left                 luma samples pY[x][y] with x=−1, y=−1, −2, are set equal                 to the reconstructed luma samples prior to the                 deblocking filter process at the locations (xTbY+x,                 yTbY+y).         -   3. The down-sampled collocated luma samples pDsY[x][y] with             x=0 . . . nTbW−1, y=0 . . . nTbH−1 are derived as follows:             -   If sps_cclm_colocated_chroma_flag is equal to 1, the                 following applies:             -   pDsY[x][y] with x=1 . . . nTbW−1, y=1 . . . nTbH−1 is                 derived as follows:                 pDsY[x][y]=(pY[2*y−1]+pY[2*x−1][2*y]+4*pY[2*x][2*y]+pY[2*x+1][2*y]+pY[2*x][2*y+1]+4)>>3  (8-163)             -   If availL is equal to TRUE, pDsY[0][y] with y=1 . . .                 nTbH−1 is derived as follows:                 pDsY[0][y]=(pY[0][2*y−1]+pY[−1][2*y]+4*pY[0][2*y]+pY[1][2*y]+pY[0][2*y+1]+4)<<3  (8-164)             -   Otherwise, pDsY[0][y] with y=1 . . . nTbH−1 is derived                 as follows:                 pDsY[0][y]=(pY[0][2*y−1]+2*pY[0][2*y]+pY[0][2*y+1]+2)>>2  (8-165)             -   If availT is equal to TRUE, pDsY[x][0] with x=1 . . .                 nTbW−1 is derived as follows:                 pDsY[x][0]=(pY[2*x][−1]+pY[2*x−1][0]+4*pY[2*x][0]+pY[2*x+1][0]+pY[2*x][1]+4)<<3  (8-166)             -   Otherwise, pDsY[x][0] with x=1 . . . nTbW−1 is derived                 as follows:                 pDsY[x][0]=(pY[2*x−1][0]+2*pY[2*x][0]+pY[2*x+1][0]+2)>>2  (8-167)             -   If availL is equal to TRUE and availT is equal to TRUE,                 pDsY[0][0] is derived as follows:                 pDsY[0][0]=(pY[0][−1]+pY[−1][0]+4*pY[0][0]+pY[1][0]+pY[0][1]+4)>>3  (8-168)             -   Otherwise if availL is equal to TRUE and availT is equal                 to FALSE, pDsY[0][0] is derived as follows:                 pDsY[0][0]=(pY[−1][0]+2*pY[0][0]+pY[1][0]+2)>>2  (8-169)             -   Otherwise if availL is equal to FALSE and availT is                 equal to TRUE, pDsY[0][0] is derived as follows:                 pDsY[0][0]=(pY[0][−1]+2*pY[0][0]+pY[0][1]+2)>>2  (8-170)             -   Otherwise (availL is equal to FALSE and availT is equal                 to FALSE), pDsY[0][0] is derived as follows:                 pDsY[0][0]=pY[0][0]  (8-171)             -   Otherwise, the following applies:                 -   pDsY[x][y] with x=1 . . . nTbW−1, y=0 . . . nTbH−1                     is derived as follows:                     pDsY[x][y]=(pY[2*x−1][2*y]+pY[2*x−1][2*y+1]+2*pY[2*x][2*y]+2*pY[2*x][2*y+1]+pY[2*x+1][2*y]+pY[2*x+1][2*y+1]+4)>>3  (8-172)                 -   If availL is equal to TRUE, pDsY[0][y] with y=0 . .                     . nTbH−1 is derived as follows:                     pDsY[0][y]=(pY[−1][2*y]+pY[−1][2*y+1]+2*pY[0][2*y]+2*pY[0][2*y+1]+pY[1][2*y]+pY[1][2*y+1]+4)>>3  (8-173)                 -   Otherwise, pDsY[0][y] with y=0 . . . nTbH−1 is                     derived as follows:                     pDsY[0][y]=(pY[0][2*y]+pY[0][2*y+1]+1)>>1  (8-174)         -   4. When numSampL is greater than 0, the selected             neighbouring left chroma samples pSelC[idx] are set equal to             p[−1][pickPosL[idx]] with idx=0 . . . (cntL−1), and the             selected down-sampled neighbouring left luma samples             pSelDsY[idx] with idx=0 . . . (cntL−1) are derived as             follows:             -   The variable y is set equal to pickPosL[idx].             -   If sps_cclm_colocated_chroma_flag is equal to 1, the                 following applies:                 -   If y>0∥availTL==TRUE,                     pSelDsY[idx]=(pY[−2][2*y−1]+pY[−3][2*y]+4*pY[−2][2*y]+pY[−1][2*y]+pY[−2][*y+1]+4)>>3  (8-175)                 -   Otherwise,                     pSelDsY[idx]=(pY[−3][0]+2*pY[−2][0]+pY[−1][0]+2)>>2  (8-177)             -   Otherwise, the following applies:                 pSelDsY[idx]=(pY[−1][2*y]+pY[−1][2*y+1]+2*pY[−2][2*y]+2*pY[−2][2*y+1]+pY[−3][2*y]+pY[−3][2*y+1]+4)>>3  (8-178)         -   5. When numSampT is greater than 0, the selected             neighbouring top chroma samples pSelC[idx] are set equal to             p[pickPosT[idx−cntL]][−1] with idx=cntL . . . (cntL+cntT−1),             and the down-sampled neighbouring top luma samples             pSelDsY[idx] with idx=cntL . . . (cntL+cntT−1) are specified             as follows:             -   The variable x is set equal to pickPosT[idx−cntL].             -   If sps_cclm_colocated_chroma_flag is equal to 1, the                 following applies:                 -   If x>0:                 -    If bCTUboundary is equal to FALSE, the following                     applies:                     pSelDsY[idx]=(pY[2*x][−3]+pY[2*x−1][−2]+4*pY[2*x][−2]+pY[2*x+1][−2]+pY[2*x][−1]+4)>>3  (8-179)                 -    Otherwise (bCTUboundary is equal to TRUE), the                     following applies:                     pSelDsY[idx]=(pY[2*x−1][−1]+2*pY[2*x][−1]+pY[2*x+1][−1]+2)>>2  (8-180)                 -   Otherwise:                 -    If availTL is equal to TRUE and bCTUboundary is                     equal to FALSE, the following applies:                     pSelDsY[idx]=(pY[0][−3]+pY[−1][−2]+4*pY[0][−2]+pY[1][−2]+pY[0][−1]+4)>>3  (8-181)                 -    Otherwise if availTL is equal to TRUE and                     bCTUboundary is equal to TRUE, the following                     applies:                     pSelDsY[idx]=(pY[−1][−1]+2*pY[0][−1]+pY[1][−1]+2)>>2  (8-182)                 -    Otherwise if availTL is equal to FALSE and                     bCTUboundary is equal to FALSE, the following                     applies:                     pSelDsY[idx]=(pY[0][−3]+2*pY[0][−2]+pY[0][−1]+2)<<2  (8-183)                 -    Otherwise (availTL is equal to FALSE and                     bCTUboundary is equal to TRUE), the following                     applies:                     pSelDsY[idx]=pY[0][−1]  (8-184)             -   Otherwise, the following applies:                 -   If x>0:                 -    If bCTUboundary is equal to FALSE, the following                     applies:                     pSelDsY[idx]=(pY[2*x−1][−2]+pY[2*x−1][−1]+2*pY[2*x][−2]+2*pY[2*x][−1]+pY[2*x+1][−2]+pY[2*x+1][−1]+4)<<3  (8-185)                 -    Otherwise (bCTUboundary is equal to TRUE), the                     following applies:                     pSelDsY[idx]=(pY[2*x−1][−1]+2*pY[2*x][−1]+pY[2*x+1][−1]+2)<<2  (8-186)                 -   Otherwise:                 -    If availTL is equal to TRUE and bCTUboundary is                     equal to FALSE, the following applies:                     pSelDsY[idx]=(pY[−1][−2]+pY[−1][−1]+2*pY[0][−2]+2*pY[0][−1]+pY[1][−2]+pY[1][−1]+4)>>3  (8-187)                 -    Otherwise if availTL is equal to TRUE and                     bCTUboundary is equal to TRUE, the following                     applies:                     pSelDsY[idx]=(pY[−1][−1]+2*pY[0][−1]+pY[1][−1]+2)>>2  (8-188)                 -    Otherwise if availTL is equal to FALSE and                     bCTUboundary is equal to FALSE, the following                     applies:                     pSelDsY[idx]=(pY[0][−2]+pY[0][−1]+1)>>1  (8-189)                 -    Otherwise (availTL is equal to FALSE and                     bCTUboundary is equal to TRUE), the following                     applies:                     pSelDsY[idx]=(pY[0][−1]  (8-190)         -   6. When cntT+cntL is not equal to 0, the variables minY;             maxY, minC and maxC are derived as follows:             -   When cntT+cntL is equal to 2, set pSelComp[3] equal to                 pSelComp[0], pSelComp[2] equal to pSelComp[1],                 pSelComp[0] equal to pSelComp[1], and pSelComp[1] equal                 to pSelComp[3], with Comp being replaced by DsY and C.             -   The arrays minGrpIdx[ ] and maxGrpIdx[ ] are set as:                 minGrpIdx[0]=0, minGrpIdx[1]=2, maxGrpIdx[0]=1,                 maxGrpIdx[1]=3.             -   If pSelDsY[minGrpIdx[0]]>pSelDsY[minGrpIdx[1]],                 Swap(minGrpIdr[0], minGrpIdx[1]),             -   If pSelDsY[maxGrpIdx[0]]>pSelDsY[maxGrpIdx[1]],                 Swap(maxGrpIdx[0], maxGrpIdx[1]).             -   If pSelDsY[minGrpIdx[0]]>pSelDsY[maxGrpIdx[1]],                 Swap(minGrpIdx, maxGrpIdx).             -   If pSelDsY[minGrpIdx[0]]>pSelDsY[maxGrpIdx[0]],                 Swap(minGrpIdx[1], maxGrpIdx[0]).             -   maxY=(pSelDsY[maxGrpIdx[0]]+pSelDsY[maxGrpIdx[1]]+1)>>1.             -   maxC=(pSelC[maxGrpIdx[0]]+pSelC[maxGrpIdx[1]]+1)>>1.             -   minY=(pSelDsY[minGrpIdx[0]]+pSelDsY[minGrpIdx[1]]+1)>>1.             -   minC=(pSelC[minGrpIdx[0]]+pSelC[minGrpIdx[1]]+1)>>1.         -   7. The variables a, b, and k are derived as follows:             -   If numSampL is equal to 0, and numSampT is equal to 0,                 the following applies:                 k=0  (8-208)                 a=0  (8-209)                 b=1<<(BitDepth_(C)−1)  (8-210)             -   Otherwise, the following applies:                 diff=maxY−minY  (8-211)             -   If diff is not equal to 0, the following applies:                 diffC=maxC−minC  (8-212)                 x=Floor(Log 2(diff))  (8-213)                 normDiff=((diff<<4)>>x)&15  (8-214)                 x+=(normDiff!=0)?1:0  (8-215)                 y=Floor(Log 2(Abs(diffC)))+1  (8-216)                 a=(diffC*(divSigTable[normDiff]18)+2^(y-1))>>y  (8-217)                 k=((3+x−y)<1)?1:3+x−y  (8-218)                 a=((3+x−y)<1)?Sign(a)*15:a  (8-219)                 b=minC−((a*minY)>>k)  (8-220)                 -   where divSigTable[ ] is specified as follows:                     divSigTable[                     ]=[0,7,6,5,5,4,4,3,3,2,2,1,1,1,1,0]  (8-221)             -   Otherwise (diff is equal to 0), the following applies:                 k=0  (8-222)                 a=0  (8-223)                 b=minC  (8-224)         -   8. The prediction samples predSamples[x][y] with x=0 . . .             nTbW−1, y=0 . . . nTbH−1 are derived as follows:             predSamples[x][y]=Clip1C(((pDsY[x][y]*a)>>k)+b)  (8-225)

2.2.3.3 Miscellaneous Intra Prediction Aspects

VTM4 includes many intra coding tools which are different from HEVC, for example, the following features have been included in the VVC test model 3 on top of the bock tree structure.

-   -   67 intra mode with wide angles mode extension     -   Block size and mode dependent 4 tap interpolation filter     -   Position dependent intra prediction combination (PDPC)     -   Cross component linear model intra prediction     -   Multi-reference line intra prediction     -   Intra sub-partitions

2.2.4 Inter Prediction in VVC 2.2.4.1 Combined Inter and Intra Prediction (CIIP)

In VTM4, when a CU is coded in merge mode, and if the CU contains at least 64 luma samples (that is, CU width times CU height is equal to or larger than 64), an additional flag is signalled to indicate if the combined inter/intra prediction (CIIP) mode is applied to the current CU.

In order to form the CIIP prediction, an intra prediction mode is first derived from two additional syntax elements. Up to four possible intra prediction modes can be used: DC, planar, horizontal, or vertical. Then, the inter prediction and intra prediction signals are derived using regular intra and inter decoding processes. Finally, weighted averaging of the inter and intra prediction signals is performed to obtain the CIIP prediction.

2.2.4.2 Miscellaneous Inter Prediction Aspects

VTM4 includes many inter coding tools which are different from HEVC, for example, the following features have been included in the VVC test model 3 on top of the bock tree structure.

-   -   Affine motion inter prediction     -   sub-block based temporal motion vector prediction     -   Adaptive motion vector resolution     -   8×8 block based motion compression for temporal motion         prediction     -   High precision ( 1/16 pel) motion vector storage and motion         compensation with 8-tap interpolation filter for luma component         and 4-tap interpolation filter for chroma component     -   Triangular partitions     -   Combined intra and inter prediction     -   Merge with MVD (MMVD)     -   Symmetrical MVD coding     -   Bi-directional optical flow     -   Decoder side motion vector refinement     -   Bi-predictive weighted averaging

2.2.5 In-Loop Filters

There are totally three in-loop filters in VTM4. Besides deblocking filter and sample adaptive offset filter (SAO), the two loop filters in HEVC, adaptive loop filter (ALF) are applied in the VTM4. The order of the filtering process in the VTM4 is the deblocking filter, SAO and ALF.

In the VTM4, the SAO and deblocking filtering processes are almost same as those in HEVC.

In the VTM4, a new process called the luma mapping with chroma scaling was added (this process was previously known as the adaptive in-loop reshaper). This new process is performed before deblocking.

2.2.6 Luma Mapping with Chroma Scaling (LMCS, Aka. In-Loop Reshaping)

In VTM4, a coding tool called the luma mapping with chroma scaling (LMCS) is added as a new processing block before the loop filters. LMCS has two main components: 1) in-loop mapping of the luma component based on adaptive piecewise linear models; 2) for the chroma components, luma-dependent chroma residual scaling is applied. FIG. 4 shows the LMCS architecture from decoder's perspective. The light-blue shaded blocks in FIG. 4 indicate where the processing is applied in the mapped domain; and these include the inverse quantization, inverse transform, luma intra prediction and adding of the luma prediction together with the luma residual. The unshaded blocks in FIG. 4 indicate where the processing is applied in the original (i.e., non-mapped) domain; and these include loop filters such as deblocking, ALF, and SAO, motion compensated prediction, chroma intra prediction, adding of the chroma prediction together with the chroma residual, and storage of decoded pictures as reference pictures. The light-yellow shaded blocks in FIG. 4 are the new LMCS functional blocks, including forward and inverse mapping of the luma signal and a luma-dependent chroma scaling process. Like most other tools in VVC, LMCS can be enabled/disabled at the sequence level using an SPS flag.

2.2.6.1 Luma Mapping with Piecewise Linear Model

The in-loop mapping of the luma component adjusts the dynamic range of the input signal by redistributing the codewords across the dynamic range to improve compression efficiency. Luma mapping makes use of a forward mapping function, FwdMap, and a corresponding inverse mapping function, InvMap. The FwdMap function is signalled using a piecewise linear model with 16 equal pieces. InvMap function does not need to be signalled and is instead derived from the FwdMap function.

The luma mapping model is signalled at the tile group level. A presence flag is signalled first. If luma mapping model is present in the current tile group, corresponding piecewise linear model parameters are signalled. The piecewise linear model partitions the input signal's dynamic range into 16 equal pieces, and for each piece, its linear mapping parameters are expressed using the number of codewords assigned to that piece. Take 10-bit input as an example. Each of the 16 pieces will have 64 codewords assigned to it by default. The signalled number of codewords is used to calculate the scaling factor and adjust the mapping function accordingly for that piece. At the tile group level, another LMCS enable flag is signalled to indicate if the LMCS process as depicted in FIG. 4 is applied to the current tile group.

Each i-th piece, i=0 . . . 15, of the FwdMap piecewise linear model is defined by two input pivot points InputPivot[ ] and two output (mapped) pivot points MappedPivot[ ].

The InputPivot[ ] and MappedPivot[ ] are computed as follows (assuming 10-bit video):

-   -   1) OrgCW=64     -   2) For i=0:16, InputPivot[i]=i*OrgCW     -   3) For i=0:16, MappedPivot[i] is calculated as follows:         -   MappedPivot[0]=0;         -   for (i=0; i<16; i++)             -   MappedPivot[i+1]=MappedPivot[i]+SignalledCW[i]                 where SignalledCW[i] is the signalled number of                 codewords for the i-th piece.

As shown in FIG. 4 , for an inter-coded block, motion compensated prediction is performed in the mapped domain. In other words, after the motion-compensated prediction block Y_(pred) is calculated based on the reference signals in the DPB, the FwdMap function is applied to map the luma prediction block in the original domain to the mapped domain, Y′_(pred)=FwdMap(Y_(pred)). For an intra-coded block, the FwdMap function is not applied because intra prediction is performed in the mapped domain. After reconstructed block Y_(r) is calculated, the InvMap function is applied to convert the reconstructed luma values in the mapped domain back to the reconstructed luma values in the original domain (Ŷ_(i)=InvMap(Y_(r))). The InvMap function is applied to both intra- and inter-coded luma blocks.

The luma mapping process (forward and/or inverse mapping) can be implemented using either look-up-tables (LUT) or using on-the-fly computation. If LUT is used, then FwdMapLUT and InvMapLUT can be pre-calculated and pre-stored for use at the tile group level, and forward and inverse mapping can be simply implemented as FwdMap(Y_(pred))=FwdMapLUT[Y_(pred)] and InvMap(Y_(r))=InvMapLUT[Y_(r)], respectively. Alternatively, on-the-fly computation may be used. Take forward mapping function FwdMap as an example. In order to figure out the piece to which a luma sample belongs, the sample value is right shifted by 6 bits (which corresponds to 16 equal pieces). Then, the linear model parameters for that piece are retrieved and applied on-the-fly to compute the mapped luma value. Let i be the piece index, a1, a2 be InputPivot[i] and InputPivot[i+1], respectively, and b1, b2 be MappedPivot[i] and MappedPivot[i+1], respectively. The FwdMap function is evaluated as follows: FwdMap(Y _(pred))=((b2−b1)/(a2−a1))*(Y _(pred-) a1)+b1

The InvMap function can be computed on-the-fly in a similar manner, except that conditional checks need to be applied instead of a simple right bit-shift when figuring out the piece to which the sample value belongs, because the pieces in the mapped domain are not equal sized.

2.2.6.2 Luma-Dependent Chroma Residual Scaling

Chroma residual scaling is designed to compensate for the interaction between the luma signal and its corresponding chroma signals. Whether chroma residual scaling is enabled or not is also signalled at the tile group level. If luma mapping is enabled and if dual tree partition (also known as separate chroma tree) is not applied to the current tile group, an additional flag is signalled to indicate if luma-dependent chroma residual scaling is enabled or not. When luma mapping is not used, or when dual tree partition is used in the current tile group, luma-dependent chroma residual scaling is disabled. Further, luma-dependent chroma residual scaling is always disabled for the chroma blocks whose area is less than or equal to 4.

Chroma residual scaling depends on the average value of the corresponding luma prediction block (for both intra- and inter-coded blocks). Denote avgY′ as the average of the luma prediction block. The value of C_(ScaleInv) is computed in the following steps:

-   -   1) Find the index Y_(Idx) of the piecewise linear model to which         avgY′ belongs based on the InvMap function.     -   2) C_(ScaleInv)=cScaleInv[Y_(Idx)], where cScaleInv[ ] is a         pre-computed 16-piece LUT.         If the current block is coded as intra, CIIP, or intra block         copy (IBC, a.k.a. current picture referencing or CPR) modes,         avgY′ is computed as the average of the intra-, CIIP-, or         IBC-predicted luma values; otherwise, avgY′ is computed as the         average of the forward mapped inter predicted luma values         (Y′_(pred) in FIG. 4 ). Unlike luma mapping, which is performed         on the sample basis, C_(ScaleInv) is a constant value for the         entire chroma block. With C_(ScaleInv), chroma residual scaling         is applied as follows:         Encoder side:C _(ResScale) =C _(ResScale) =C _(Res) *C _(Scale)         =C _(Res) /C _(scaleInv)         Decoder side: C _(Res) =C _(ResScale) /C _(Scale) =C _(ResScale)         *C _(ScaleInv)

2.2.6.3 Corresponding Working Draft in JVET-M1001_v7 with the Adoption in JVET-N0220

The following spec is based on the modified working draft of JVET-M1001 and the adoption in JVET-N0220. The modification in the adopted JVET-N0220 is shown in bold and underlining.

Syntax Tables

In 7.3.2.1 Sequence parameter set RBSP syntax

In 7.3.4.1 General tile group header syntax

if( NumTilesInCurrTileGroup > 1 ) {

ue(v)  for( i = 0; i < NumTilesInCurrTileGroup − 1; i++ )

u(v) }

In 7.3.4.4 Luma mapping with chroma scaling data syntax

—

Semantics

In 7.4.3.1 Sequence parameter set RBSP semantics

sps_lmcs_enabled_flag equal to 1 specifies that luma mapping with chroma scaling is used in the CVS. sps_lmcs_enabled_flag equal to 0 specifies that luma mapping with chroma scaling is not used in the CVS.

tile_group_lmcs_model_present_flag equal to 1 specifies that lmcs_data( ) is present in the tile group header. tilegroup_lmcs_model_present_flag equal to 0 specifies that lmcs_data( ) is not present in the tile group header. When tilegroup_lmcs_model_present_flag is not present, it is inferred to be equal to 0.

tile_group_lmcs_enabled_flag equal to 1 specifies that luma mappin with chroma scaling is enabled for the current tile group. tilegroup_lmcs_enabled_flag equal to 0 specifies that luma mapping with chroma scaling is not enabled for the current tile group. When tile_group_lmcs_enabled_flag is not present, it is inferred to be equal to 0.

tile_group_chroma_residual_scale_flag equal to 1 specifies that chroma residual scaling is enabled for the current tile group. tile_group_chroma_residual_scale_flag equal to 0 specifies that chroma residual scaling is not enabled for the current tile group. When the group_chroma_residual_scale_flag is not present, it is inferred to be equal to 0.

In 7.4.5.4 Luma mapping with chroma scaling data semantics

lmcs_min_bin_idx specifies the minimum bin index used in the luma mapping with chroma scaling construction process. The value of lmcs_min_bin_idx shall be in the range of 0 to 15, inclusive.

lmcs_delta_max_bin_idx specifies the delta value between 15 and the maximum bin index LmcsMaxBinIdx used in the luma mapping with chroma scaling construction process. The value of lmcs_delta_max_bin_idx shall be in the range of 0 to 15, inclusive. The value of LmcsMaxBinIdx is set equal to 15 lmcs_delta_max_bin_idx. The value of LmcsMaxBinIdx shall be larger than or equal to lmcs_min_bin_idx.

lmcs_delta_cw_prec_minus1 plus 1 specifies the number of bits used for the representation of the syntax lmcs_delta_abs_cw[i]. The value of lmcs_delta_cw_prec_minus1 shall be in the range of 0 to BitDepthY−2, inclusive.

lmcs_delta_abs_cw[i] specifies the absolute delta codeword value for the ith bin.

lmcs_delta_sign_cw_flag[i] specifies the sign of the variable lmcsDeltaCW[i] as follows:

-   -   If lmcs_delta_sign_cw_flag[i] is equal to 0, lmcsDeltaCW[i] is a         positive value.     -   Otherwise (lmcs_delta_sign_cw_flag[i] is not equal to 0),         lmcsDeltaCW[i] is a negative value.

When lmcs_delta_sign_cw_flag[i] is not present, it is inferred to be equal to 0.

The variable OrgCW is derived as follows: OrgCW=(1<<BitDepth_(Y))/16  (7-70)

The variable lmcsDeltaCW[i], with i=lmcs_min_bin_idx . . . LmcsMaxBinIdx, is derived as follows: lmcsDeltaCW[i]=(1−2*lmcs_delta_sign_cw_flag[i])*lmcs_delta_abs_cw[i]  (7-71)

The variable lmcsCW[i] is derived as follows:

-   -   For i=0 . . . lmcs_min_bin_idx 1, lmcsCW[i] is set equal 0.     -   For i=lmcs_min_bin_idx . . . LmcsMaxBinIdx, the following         applies:         lmcsCW[i]=OrgCW+lmcsDeltaCW[i]  (7-72)

The value of lmcsCW[i] shall be in the range of (OrgCW>>3) to (OrgCW<<3−1), inclusive.

-   -   For i=LmcsMaxBinIdx+1 . . . 15, lmcsCW[i] is set equal 0.

It is a requirement of bitstream conformance that the following condition is true: Σ_(i=0) ¹⁵lmcsCW[i]<=(1<<BitDepth_(Y))−1  (7-73)

The variable InputPivot[i], with i=0 . . . 16, is derived as follows: InputPivot[i=i]*OrgCW  (7-74)

The variable LmcsPivot[i] with i=0 . . . 16, the variables ScaleCoeff[i] and InvScaleCoeff[i] with i=0 . . . 15, are derived as follows:

LmcsPivot[ 0 ] = 0; for( i = 0; i <= 15; i++ ) { LmcsPivot[ i + 1 ] = LmcsPivot[ i ] + lmcsCW[ i ]

                                if( lmcsCW[ i ] = = 0 )                                  InvScaleCoeff[ i ] = 0                                          else                    

}

The variable ChromaScaleCoeff[i], with i=0 . . . 15, is derived as follows:

if ( lmcsCW[ i ] = = 0 )                 ChromaScaleCoeff[ i ] = (1 << 11) else {            

}

The variables ClipRange, LmcsMinVal, and LmcsMaxVal are derived as follows: ClipRange=((lmcs_min_bin_idx>0)&&(LmcsMaxBinIdx<15)  (7-77) LmcsMinVal=16<<(BitDepth_(Y)−8)  (7-78) LmcsMaxVal=235<<(BitDepth_(Y)−8)  (7-79)

-   -   NOTE Arrays InputPivot[i] and LmcsPivot[i], ScaleCoeff[i], and         InvScaleCoeff[i], ChromaScaleCoeff[i], ClipRange, LmcsMinVal and         LmcsMaxVal, are updated only when         tile_group_lmcs_model_present_flag is equal to 1. Thus, the lmcs         model may be sent with an IRAP picture, for example, but lmcs is         disabled for that IRAP picture.

3. Drawbacks of Existing Implementations

The current design of LMCS/CCLM may have the following problems:

-   -   1. In LMCS coding tool, the chroma residual scaling factor is         derived by the average value of the collocated luma prediction         block, which results in a latency for processing the chroma         samples in LMCS chroma residual scaling.         -   a) In case of single/shared tree, the latency is caused             by (a) waiting for all the prediction samples of the whole             luma block available, and (b) averaging all the luma             prediction samples obtained by (a).         -   b) In case of dual/separate tree, the latency is even worse             since separate block partitioning structure for luma and             chroma components is enabled in I slices. Therefore, one             chroma block may correspond to multiple luma blocks, and one             4×4 chroma block may correspond to a 64×64 luma block. Thus             the worst case is that the chroma residual scaling factor of             current 4×4 chroma block may need to wait until all the             prediction samples in the whole 64×64 luma block are             available. In a word, the latency issue in dual/separate             tree would be much more serious.     -   2. In CCLM coding tool, the CCLM model computation for intra         chroma prediction depends on the left and above reference         samples of both luma block and chroma block. And the CCLM         prediction for a chroma block depends on the collocated luma         reconstructed samples of the same CU. This would cause high         latency in dual/separate tree.         -   In case of dual/separate tree, one 4×4 chroma block may             correspond to a 64×64 luma block. Thus the worst case is             that the CCLM process for the current chroma block may need             wait until the corresponding whole 64×64 luma block being             reconstructed. This latency issue is similar as LMCS chroma             scaling in dual/separate tree.

4. Example Techniques and Embodiments

To tackle the problems, we propose several methods to remove/reduce/restrict the cross-component dependency in luma-dependent chroma residual scaling, CCLM, and other coding tools that rely on information from a different colour component.

The detailed embodiments described below should be considered as examples to explain general concepts. These embodiments should not be interpreted narrowly way. Furthermore, these embodiments can be combined in any manner.

It is noted that although the bullets described below explicitly mention LMCS/CCLM, the methods may be also applicable to other coding tools that rely on information from a different colour component. In addition, the term ‘luma’ and ‘chroma’ mentioned below may be replaced by ‘a first color component’ and ‘a second color component’ respectively, such as ‘G component’ and ‘B/R component’ in the RGB color format.

In the following discussion, the definition a “collocated sample/block” aligns with the definition of collocated sample/block in VVC working draft JVET-M1001. To be more specific, in 4:2:0 colour format, suppose the top-left sample of a chroma block is at position (xTbC, yTbC), then the top-left sample of the collocated luma block location (xTbY, yTbY) is derived as follows: (xTbY, yTbY)=(xTbC<<1, yTbC<<1). As illustrated in FIG. 5 , the top-left sample of the current chroma block is located at (x=16,y=16) in the chroma picture, then the top-left sample of its collocated luma block is located at (x=32,y=32) in the luma picture, regardless of the block partition of collocated luma block in the luma picture. For another example, saying in the same color component, the location of the top-left sample of the collocated block in the reference frame should be same with the location of the top-left sample of the current block in the current frame, as illustrated in FIG. 6 , suppose the top-left sample of the current block is (x,y) in the current frame, then the top-left left sample of the collocated block of the current block have the same location (x,y) in the reference frame.

In the following discussion, a “corresponding block” may have different location with the current block. For an example, there might be a motion shift between the current block and its corresponding block in the reference frame. As illustrated in FIG. 6 , suppose the current block is located at (x,y) in the current frame and it has a motion vector (mv_(x), mv_(y)), then a corresponding block of the current block may be located at (x+mv_(x),y+mv_(y)) in the reference frame. And for an IBC coded block, the collocated luma block (pointed by zero vector) and the corresponding luma block (pointed by non-zero-BV) may locate in different places of the current frame. For another example, when the partition of luma block doesn't align with the partition of chroma block (in dual tree partition of I slices), the collocated luma block of the current chroma block may belong to a larger luma block which depends on the partition size of the overlapped luma coding block covering the top-left sample of the collocated luma block. As illustrated in FIG. 5 , assume the bold rectangle denotes the partitions of the block, so that a 64×64 luma block is firstly split by an BT and then the right part of the 64×64 luma block is further split by a TT, which results in three luma blocks of sizes equal to 32×16, 32×32, 32×16, respectively. Thus looking at the top-left sample (x=32, y=32) of the collocated luma block of the current chroma block, it belongs to the center 32×32 luma block of the TT partition. In this case, we call the corresponding luma block that covers the top-left sample of the collocated luma block as a “corresponding luma block”. Therefore, in this example, the top-left sample of the corresponding luma block is located at (x=32, y=16).

Hereinafter, DMVD (decoder-side motion vector derivation) is used to represent BDOF (a.k.a BIO) or/and DMVR (decode-side motion vector refinement) or/and FRUC (frame rate up-conversion) or/and other method that refines motion vector or/and prediction sample value at decoder.

Removal of the Chroma Scaling Latency of LMCS and Model Computation of CCLM

-   -   1. It is proposed that for an inter-coded block, one or multiple         reference samples of the current block in reference frames may         be used to derive the chroma residual scaling factor in the LMCS         mode.         -   a) In one example, reference luma samples may be directly             used to derive the chroma residual scaling factor.             -   i. Alternatively, interpolation may be firstly applied                 to reference samples and the interpolated samples may be                 used to derive the chroma residual scaling factor.             -   ii. Alternatively, reference samples in different                 reference frames may be utilized to derive the final                 reference samples that are used for the chroma residual                 scaling factor derivation.                 -   1) In one example, for bi-prediction coded blocks,                     the above method may be applied.             -   iii. In one example, the intensities of reference                 samples may be converted to reshaping domain before                 being used to derive the chroma residual scaling factor.             -   iv. In one example, linear combination of the reference                 samples may be used to derive the chroma residual                 scaling factor.                 -   1) For example, a×S+b may be used to derive the                     chroma residual scaling factor, where S is a                     reference sample, a and b are parameters. In one                     example, a and b may be derived by Localized                     Illuminate Compensation (LIC).         -   b) In one example, the location of the reference luma             samples in the reference frame may depend on the current             block's motion vector(s).             -   i. In one example, a reference sample belongs to a                 reference luma block, which is in a reference picture,                 and with the same width and height as the current luma                 block. The position of the reference luma sample in the                 reference picture may be calculated as the position of                 its corresponding luma sample in the current picture,                 adding a motion vector.             -   ii. In one example, the position of the reference luma                 samples may be derived by the position of top-left (or                 center, or bottom-right) sample of the current luma                 block and current block's motion vector, referred as a                 corresponding luma sample in the reference frame.                 -   1) In one example, an integer motion vector may be                     used to derive the corresponding luma sample in the                     reference frame. In one example, the motion vector                     associated with one block may be either rounded                     toward zero, or rounded away from zero to derive the                     integer motion vector.                 -   2) Alternatively, a fractional motion vector may be                     used to derive the corresponding luma sample in the                     reference frame, so that the interpolation process                     may be required to derive the fractional reference                     samples.             -   iii. Alternatively, the position of the reference luma                 samples may be derived by the position of top-left (or                 center, or bottom-right) sample of current luma block.             -   iv. Alternatively, multiple corresponding luma samples                 at some pre-defined positions in the reference frame may                 be picked to calculate the chroma residual scaling                 factor.         -   c) In one example, the median or average value of the             multiple reference luma samples may be used to derive the             chroma residual scaling factor.         -   d) In one example, the reference luma samples in pre-defined             reference frames may be used to derive the chroma residual             scaling factor.             -   i. In one example, the pre-defined reference frame may                 be the one with reference index equal to 0 of reference                 picture list 0.             -   ii. Alternatively, the reference index and/or reference                 picture list for the pre-defined reference frame may be                 signaled in sequence/picture/tile group/slice/tile/CTU                 row/video unit level.             -   iii. Alternatively, the reference luma samples in                 multiple reference frames may be derived and the                 averaged or weighted average values may be utilized to                 get the chroma residual scaling factor.     -   2. It is proposed that whether and how to derive the chroma         residual scaling factor from luma samples in the LMCS mode may         depend on whether the current block applies bi-prediction.         -   a) In one example, the chroma residual scaling factor is             derived for each prediction direction individually.     -   3. It is proposed that whether and how to derive the chroma         residual scaling factor from luma samples in the LMCS mode may         depend on whether the current block applies sub-block-based         prediction.         -   a) In one example, the sub-block-based prediction is affine             prediction;         -   b) In one example, the sub-block-based prediction is             Alternative Temporal Motion Vector Prediction (ATMVP).         -   c) In one example, the chroma residual scaling factor is             derived for each sub-block individually.         -   d) In one example, the chroma residual scaling factor is             derived for the whole block even if it is predicted by             sub-blocks.             -   i. In one example, motion vector of one selected                 sub-block (e.g., top-left sub-block) may be used to                 identify the reference samples of current block as                 described in bullet 1.     -   4. It is proposed that the luma prediction values used to derive         the chroma residual scaling factor may be intermediate luma         prediction value instead of the final luma prediction value.         -   a) In one example, the luma prediction values before the             process of Bi-Directional Optical Flow (BDOF, a.k.a. BIO)             may be used to derive the chroma residual scaling factor.         -   b) In one example, the luma prediction values before the             process of Decoder-side Motion Vector Refinement (DMVR) may             be used to derive the chroma residual scaling factor.         -   c) In one example, the luma prediction values before the             process of LIC may be used to derive the chroma residual             scaling factor.         -   d) In one example, the luma prediction values before the             process of Prediction Refinement Optical Flow (PROF) as             proposed in JVET-N0236 may be used to derive the chroma             residual scaling factor.     -   5. Intermediate motion vectors may be used to identify the         reference samples.         -   a) In one example, motion vector before the process of BDOF             or/and DMVR or/and other DMVD methods may be used to             identify the reference samples.         -   b) In one example, the motion vector before the process of             Prediction Refinement Optical Flow (PROF) as proposed in             JVET-N0236 may be used to identify the reference samples.     -   6. The above methods may be applicable when the current block is         coded with inter mode.     -   7. It is proposed that for an IBC-coded block, one or multiple         reference samples in reference block of current frame may be         used to derive the chroma residual scaling factor in the LMCS         mode. When the block IBC-coded, the term “motion vector” may         also be referred as “block vector”, where the reference picture         is set as the current picture.         -   a) In one example, a reference sample belongs to a reference             block, which is in the current picture, and with the same             width and height as the current block. The position of the             reference sample may be calculated as the position of its             corresponding sample adding a motion vector.         -   b) In one example, the position of the reference luma             samples may be derived by the position of top-left (or             center, or bottom-right) sample of current luma block adding             a motion vector.         -   c) Alternatively, the position of the reference luma samples             may be derived by the position of top-left (or center, or             bottom-right) sample of current luma block adding current             block's block vector.         -   d) Alternatively, multiple corresponding luma samples at             some pre-defined positions in the reference region of             current luma block may be picked to calculate the chroma             residual scaling factor.         -   e) In one example, multiple corresponding luma samples may             be computed with a function to derive the chroma residual             scaling factor.             -   i. For example, the median or average value of multiple                 corresponding luma samples may be computed to derive the                 chroma residual scaling factor.         -   f) In one example, the intensities of reference samples may             be converted to reshaping domain before being used to derive             the chroma residual scaling factor.             -   i. Alternatively, the intensities of reference samples                 may be converted to original domain before being used to                 derive the chroma residual scaling factor     -   8. It is proposed that one or multiple prediction/reconstructed         samples which are located at the identified location(s) of the         current luma block in the current frame may be used to derive         the chroma residual scaling factor for the current chroma block         in the LMCS mode.         -   a) In one example, if current block is inter-coded, the luma             prediction (or reconstruction) sample located in the center             of the current luma block may be picked to derive the chroma             residual scaling factor.         -   b) In one example, the average value of the first M×N luma             prediction (or reconstruction) samples may be picked to             derive the chroma residual scaling factor, where M×N could             be smaller than collocated luma block size width×height.     -   9. It is proposed that the whole or partial of the procedure         used to calculate the CCLM model may be used for the chroma         residual scaling factor derivation of current chroma block in         the LMCS mode.         -   a) In one example, reference samples which are located at             the identified locations of neighboring luma samples of the             collocated luma block in CCLM model parameter derivation             process may be utilized to derive chroma residual scaling             factor.             -   i. In one example, those reference samples may be                 directly used.             -   ii. Alternatively, downsampling may be applied to those                 reference samples, and downsampled reference samples may                 be applied.         -   b) In one example, K out of S reference samples selected for             CCLM model computation may be used for chroma residual             scaling factor derivation in the LMCS mode. E.g., K is equal             to 1 and S is equal to 4.         -   c) In one example, the average/minimum/maximum value of the             reference samples of the collocated luma block in CCLM mode             may be used for chroma residual scaling factor derivation in             the LMCS mode.     -   10. How to select samples for derivation of chroma residual         scaling factors may be dependent on the coded information of         current block.         -   a) The coded information may include QP, coding mode, POC,             intra-prediction mode, motion information and so on.         -   b) In one example, for IBC coded or Non-IBC coded blocks,             the way to select samples may be different.         -   c) In one example, the way to select samples may be             different based on the reference picture information, such             as POC distance between reference pictures and current             picture.     -   11. It is proposed that the chroma residual scaling factor         and/or the model computation of CCLM may depend on neighboring         samples of a corresponding luma block which covers the top-left         sample of the collocated luma block.         -   a) The “corresponding luma coding block” may be defined as             the coding block which covers the top-left position of the             collocated luma coding block.             -   i. FIG. 5 shows an example, where for an intra-coded                 chroma block in dual tree case, the CTU partition of                 chroma component may be different from the CTU partition                 of luma component. Firstly, a “corresponding luma coding                 block” covering the top-left sample of the collocated                 luma block of current chroma block is retrieved. Then by                 using the block size information of the “corresponding                 luma coding block”, the top-left sample of the                 “corresponding luma coding block” can be derived, the                 top-left luma sample of the “corresponding luma coding                 block” covering the top-left sample of the collocated                 luma block is located at (x=32, y=16).         -   b) In one example, the reconstructed samples not in the             “corresponding luma coding block” may be used to derive the             chroma residual scaling factor and/or model computation of             CCLM.             -   i. In one example, the reconstructed samples adjacent to                 the “corresponding luma coding block” may be used to                 derive the chroma residual scaling factor and/or model                 computation of CCLM.                 -   1) In one example, N samples located at the left                     neighboring columns and/or the above neighboring                     rows of the “corresponding luma coding block” may be                     used to derive the chroma residual scaling factor                     and/or the model computation of CCLM, where N=1 . .                     . 2W+2H, W and H are the width and height of the                     “corresponding luma coding block”.                 -    a) Suppose the top-left sample of the                     “corresponding luma coding block” is (xCb, yCb),                     then in one example, the above neighboring luma                     sample may locate at (xCb+W/2, yCb−1), or (xCb−1,                     yCb−1). In an alternative example, the left                     neighboring luma sample may locate at (xCb+W−1,                     yCb−1).                 -    b) In one example, the location(s) of the                     neighboring sample(s) may be fixed, and/or in a                     pre-defined checking order.             -   2) In one example, 1 out of N neighboring samples may be                 selected to derive the chroma residual scaling factor                 and/or the model computation of CCLM. Assume N=3, and                 the checking order of the three neighbor samples (xCb−1,                 yCb−H−1), (xCb+W/2, yCb−1), (xCb−1, yCb−1), then the                 first available neighboring sample in the checking list                 may be selected to derive the chroma residual scaling                 factor.             -   3) In one example, the median or average value of N                 samples located at the left neighboring columns and/or                 the above neighboring rows of the “corresponding luma                 coding block” may be used to derive the chroma residual                 scaling factor and/or the model computation of CCLM,                 where N=1 . . . 2W+2H, W and H are the width and height                 of the “corresponding luma coding block”.         -   c) In one example, whether to perform the chroma residual             scaling may depend on the “available” neighbouring samples             of a corresponding luma block.             -   i. In one example, the “availability” of neighbouring                 samples may depend on the encoding mode of the current                 block/sub-block or/and encoding mode of the neighbouring                 sample.                 -   1) In one example, for a block coded in inter mode,                     neighbouring samples coded in intra mode or/and IBC                     mode or/and CIIP mode or/and LIC mode may be                     considered as “unavailable”.                 -   2) In one example, for a block coded in inter mode,                     neighbouring samples employs diffusion filter or/and                     bilateral filter or/and Hadamard transform filter                     may be considered as “unavailable”.             -   ii. In one example, the “availability” of the                 neighbouring samples may depend on the width and/or                 height of the current picture/tile/tile                 group/VPDU/slice.                 -   1) In one example, if the neighbouring block locates                     outside the current picture, then it is treated as                     “unavailable”.             -   iii. In one example, when there is no “available”                 neighbouring sample, chroma residual scaling may be                 disallowed.             -   iv. In one example, when the number of “available”                 neighbouring samples is smaller than K (K>=1), chroma                 residual scaling may be disallowed.             -   v. Alternatively, the unavailable neighbouring sample                 may be filled by a default fixed value, or padding, or                 substitution, so that the chroma residual scaling may                 always be applied.                 -   1) In one example, if the neighbouring sample is not                     available, then it may be filled by 1<<(bitDepth−1),                     where bitDepth specifies the bit depth of the                     samples of the luma/chroma components.                 -   2) Alternatively, if the neighbouring sample is not                     available, then it may be filled by padding from the                     surrounding samples located in the                     left/right/top/bottom neighbour.                 -   3) Alternatively, if the neighbouring sample is not                     available, then it may be substituted by the first                     available adjacent sample at a pre-defined checking                     order.         -   d) In one example, the filtered/mapped reconstructed samples             neighboring the “corresponding luma coding block” may be             used to derive the chroma residual scaling factor and/or the             model computation of CCLM.             -   i. In one example, the filtering/mapping process may                 include reference smoothing filtering for intra blocks,                 post-filtering such as bilateral filter, Hadamard                 transform based filter, forward mapping of reshaper                 domain and so on.                 Restriction on Whether Chroma Residual Scaling and/or                 CCLM is Applied or Not     -   12. It is proposed that whether the chroma residual scaling or         CCLM is applied or not may depend on the partition of the         corresponding and/or the collocated luma block.         -   a) In one example, whether to enable or disable tools with             cross-component information may depend on the number of             CU/PU/TUs within the collocated luma (e.g., Y or G             component) block.             -   i. In one example, if the number of CU/PU/TUs within the                 collocated luma (e.g., Y or G component) block exceeds a                 number threshold, such tools may be disabled.             -   ii. Alternatively, whether to enable or disable tools                 with cross-component information may depend on the                 partition tree depth.                 -   1) In one example, if the maximum (or minimum or                     average or other variation) quadtree depth of CUs                     within the collocated luma block exceeds a                     threshold, such tools may be disabled.                 -   2) In one example, if the maximum (or minimum or                     average or other variation) BT and/or TT depth of                     CUs within the collocated luma block exceeds a                     threshold, such tools may be disabled.             -   iii. Alternatively, furthermore, whether to enable or                 disable tools with cross-component information may                 depend on the block dimension of the chroma block.             -   iv. Alternatively, furthermore, whether to enable or                 disable tools with cross-component information may                 depend on whether the collocated luma cross multiple                 VPDUs/pre-defined region sizes.             -   v. The thresholds in the above discussion may be fixed                 numbers, or may be signaled, or may be dependent on                 standard profiles/levels/tiers.         -   b) In one example, if the collocated luma block of current             chroma block is divided by multiple partitions (e.g., in             FIG. 7 ), then the chroma residual scaling and/or CCLM may             be prohibited.             -   i. Alternatively, if the collocated luma block of                 current chroma block is not split (e.g., within one                 CU/TU/PU), then the chroma residual scaling and/or CCLM                 may be applied.         -   c) In one example, if the collocated luma block of current             chroma block contains more than M CUs/PUs/TUs, then the             chroma residual scaling and/or CCLM may be prohibited.             -   i. In one example, M may be an integer greater than 1.             -   ii. In one example, M may be dependent on whether it is                 CCLM or chroma residual scaling process.             -   iii. M may be fixed numbers, or may be signaled, or may                 be dependent on standard profiles/levels/tiers         -   d) The above-mentioned CUs within the collocated luma block             may be interpreted to be all CUs within the collocated luma             block. Alternatively, CUs within the collocated luma block             may be interpreted to be partial CUs within the collocated             luma block, such as CUs along the boundary of the collocated             luma block.         -   e) The above-mentioned CUs within the collocated luma block             may be interpreted to be sub-CUs or sub-blocks.             -   i. For example, sub-CUs or sub-blocks may be used in                 ATMVP;             -   ii. For example, sub-CUs or sub-blocks may be used in                 affine prediction;             -   iii. For example, sub-CUs or sub-blocks may be used in                 Intra Sub-Partitions (ISP) mode.         -   f) In one example, if the CU/PU/TU covering the top-left             luma sample of the collocated luma block is larger than a             pre-defined luma block size, then the chroma residual             scaling and/or CCLM may be prohibited.             -   i. An example is depicted in FIG. 8 , the collocated                 luma block is 32×32 but it is within a corresponding                 luma block with size equal to 64×64, then if the                 pre-defined luma block size is 32×64, the chroma                 residual scaling and/or CCLM is prohibited in this case             -   ii. Alternatively, if the collocated luma block of                 current chroma block is not split, and the corresponding                 luma block covering the top-left luma sample of the                 collocated luma blocks is completely included within a                 pre-defined bounding box, then the chroma residual                 scaling and/or CCLM for current chroma block may be                 applied. The bounding box may be defined as a rectangle                 with width W and height H, denoted by W×H, as shown in                 FIG. 9 , where the corresponding luma block is with                 width 32 and height 64, and the bounding box is with                 width 40 and height 70.                 -   1) In one example, the size W×H of the bounding box                     may be defined according to the CTU width and/or                     height, or according to the CU width and/or height,                     or according to arbitrary values.         -   g) In one example, if the collocated luma block of current             chroma block is divided by multiple partitions, then only             the prediction samples (or reconstructed samples) inside the             pre-defined partition of the collocated luma block are used             to derive the chroma residual scaling factor in LMCS mode.             -   i. In one example, the average of all the prediction                 samples (or reconstructed samples) in the first                 partition of the collocated luma block are used to                 derive the chroma residual scaling factor in the LMCS                 mode.             -   ii. Alternatively, the top-left prediction sample (or                 reconstructed sample) in the first partition of the                 collocated luma block is used to derive the chroma                 residual scaling factor in the LMCS mode.             -   iii. Alternatively, the center prediction sample (or                 reconstructed sample) in the first partition of the                 collocated luma block is used to derive the chroma                 residual scaling factor in the LMCS mode.         -   h) It is proposed that whether and how to apply the             cross-component tools such as CCLM and LMCS may depend on             the coding mode(s) of one or multiple luma CUs which cover             at least one sample of the collocated luma block.             -   i. For example, the cross-component tools are disabled                 if one or multiple luma CUs which cover at least one                 sample of the collocated luma block are coded with                 affine mode;             -   ii. For example, the cross-component tools are disabled                 if one or multiple luma CUs which cover at least one                 sample of the collocated luma block are coded with                 bi-prediction;             -   iii. For example, the cross-component tools are disabled                 if one or multiple luma CUs which cover at least one                 sample of the collocated luma block are coded with BDOF;             -   iv. For example, the cross-component tools are disabled                 if one or multiple luma CUs which cover at least one                 sample of the collocated luma block are coded with DMVR;             -   v. For example, the cross-component tools are disabled                 if one or multiple luma CUs which cover at least one                 sample of the collocated luma block are coded with                 matrix affine prediction mode as proposed in JVET-N0217;             -   vi. For example, the cross-component tools are disabled                 if one or multiple luma CUs which cover at least one                 sample of the collocated luma block are coded with inter                 mode;             -   vii. For example, the cross-component tools are disabled                 if one or multiple luma CUs which cover at least one                 sample of the collocated luma block are coded with ISP                 mode;             -   viii. In one example, “one or multiple luma CUs which                 cover at least one sample of the collocated luma block”                 may refer the corresponding luma block.         -   i) When CCLM/LMCS is prohibited, signalling of the             indication of usage of CCLM/LMCS may be skipped.         -   j) In this disclosure, CCLM may refer to any variants modes             of CCLM, including LM mode, LM-T mode, and LM-L mode.     -   13. It is proposed that whether and how to apply the         cross-component tools such as CCLM and LMCS may be performed on         part of a chroma block.         -   a) In one example, whether and how to apply the             cross-component tools such as CCLM and LMCS at chroma             subblock level.             -   i. In one example, a chroma subblock is defined as a 2×2                 or 4×4 block in a chroma CU.             -   ii. In one example, for a chroma subblock, when the                 corresponding luma coding block of the current chroma CU                 covers all samples of the corresponding block of the                 subblock, CCLM may be applied.             -   iii. In one example, for a chroma subblock, when not all                 samples of the corresponding block are covered by the                 corresponding luma coding block of the current chroma                 CU, CCLM is not applied.             -   iv. In one example, the parameters of CCLM or LMCS are                 derived for each chroma subblock as treating the                 subblock as a chroma CU.             -   v. In one example, when CCLM or LMCS are applied for a                 chroma subblock, the collocated block's samples may be                 used.                 Applicability of Chroma Residual Scaling in LMCS Mode     -   14. It is proposed that whether luma dependent chroma residual         scaling can be applied may be signalled at other syntax level in         addition to the tile group header as specified in JVET-M1001.         -   a) For example, a chroma_residual_scale_flag may be             signalled at sequence level (e.g. in SPS), at picture level             (e.g. in PPS or picture header), at slice level (e.g. in             slice header), at tile level, at CTU row level, at CTU             level, at CU level. chroma_residual_scale_flag equal to 1             specifies that chroma residual scaling is enabled for the             CUs below the signalled syntax level.             chroma_residual_scale_flag equal to 0 specifies that chroma             residual scaling is not enabled for below the signalled             syntax level. When chroma_residual_scale_flag is not             present, it is inferred to be equal to 0.         -   b) In one example, if chroma residual scaling is constrained             at a partition node level. Then chroma_residual_scale_flag             may not be signalled and inferred to be 0 for CUs covered by             the partition node. In one example, a partition node may be             a CTU (CTU is treated as the root node of quaternary tree             partition).         -   c) In one example, if chroma residual scaling is constrained             for chroma block size equal or smaller than 32×32, then             chroma_residual_scale_flag may not be signalled and inferred             to be 0 for chroma block size equal or smaller than 32×32.             Applicability of CCLM Mode     -   15. It is proposed that whether CCLM mode can be applied may be         signalled at other syntax levels in addition to the sps level as         specified in JVET-M1001.         -   a) For example, it may be signalled at picture level (e.g.             in PPS or picture header), at slice level (e.g. in slice             header), at tile group level (e.g. in tile group header), at             tile level, at CTU row level, at CTU level, at CU level.         -   b) In one example, cclm_flag may not be signalled and             inferred to be 0 if CCLM cannot be applied.             -   i. In one example, if chroma residual scaling is                 constrained for chroma block size equal or smaller than                 8×8, then cclm_flag may not be signalled and inferred to                 be 0 for chroma block size equal or smaller than 8×8.                 Unification of Chroma Residual Scaling Factor Derivation                 for Intra Mode and Inter Mode     -   16. Chroma residual scaling factor may be derived after         encoding/decoding a luma block and may be stored and used for         following coded blocks.         -   a) In one example, certain prediction samples or/and             intermediate prediction samples or/and reconstructed samples             or/and reconstructed samples before loop filtering (e.g.,             before processed by deblocking filter or/and SAO filter             or/and bilateral filter or/and Hadamard transform filter             or/and ALF filter) in the luma block may be used for             derivation of the chroma residual scaling factor.             -   i. For example, partial samples in the bottom row or/and                 right column of the luma block may be used for                 derivation of the chroma residual scaling factor.         -   b) In single tree case, when encoding a block coded in intra             mode or/and IBC mode or/and inter mode, derived chroma             residual scaling factor of neighboring blocks may be used             for deriving scaling factor of the current block.             -   i. In one example, certain neighboring blocks may be                 checked in order, and the first available chroma                 residual scaling factor may be used for the current                 block.             -   ii. In one example, certain neighboring blocks may be                 checked in order, and a scaling factor may be derived                 based on the first K available neighboring chroma                 residual scaling factors.             -   iii. In one example, for a block coded in inter mode                 or/and CIIP mode, if a neighboring block is coded in                 intra mode or/and IBC mode or/and CIIP mode, chroma                 residual scaling factor of the neighboring block may be                 considered as “unavailable”.             -   iv. In one example, neighboring blocks may be checked in                 order of left (or above left)→above (or above right).                 -   1) Alternatively, neighboring blocks may be checked                     in order of above (or above right)→left (or above                     left).         -   c) In separate tree case, when encoding a chroma block, the             corresponding luma block may be first identified. Then,             derived chroma residual scaling factor of its (e.g., the             corresponding luma block) neighboring blocks may be used for             deriving scaling factor of the current block.             -   i. In one example, certain neighboring blocks may be                 checked in order, and the first available chroma                 residual scaling factor may be used for the current                 block.             -   ii. In one example, certain neighboring blocks may be                 checked in order, and a scaling factor may be derived                 based on the first K available neighboring chroma                 residual scaling factors.         -   d) Neighboring blocks may be checked in a predefined order.             -   i. In one example, neighboring blocks may be checked in                 order of left (or above left)→above (or above right)             -   ii. In one example, neighboring blocks may be checked in                 order of above (or above right)→left (or above left).             -   iii. In one example, neighboring blocks may be checked                 in order of below left→left→above right→above→above                 left.             -   iv. In one example, neighboring blocks may be checked in                 order of left→above→above right→below left→above left.         -   e) In one example, whether to apply chroma residual scaling             may depend on the “availability” of neighbouring block.             -   i. In one example, when there is no “available”                 neighbouring block, chroma residual scaling may be                 disallowed.             -   ii. In one example, when the number of “available”                 neighbouring blocks is smaller than K (K>=1), chroma                 residual scaling may be disallowed.             -   iii. Alternatively, when there is no “available”                 neighbouring block, chroma residual scaling factor may                 be derived by a default value.                 -   1) In one example, a default value 1<<(BitDepth−1)                     may be used to derive the chroma residual scaling                     factor.

5. Example Implementations of the Disclosed Technology

FIG. 10 is a block diagram of a video processing apparatus 1000. The apparatus 1000 may be used to implement one or more of the methods described herein. The apparatus 1000 may be embodied in a smartphone, tablet, computer, Internet of Things (IoT) receiver, and so on. The apparatus 1000 may include one or more processors 1002, one or more memories 1004 and video processing hardware 1006. The processor(s) 1002 may be configured to implement one or more methods (including, but not limited to, methods 800 and 900) described in the present document. The memory (memories) 1004 may be used for storing data and code used for implementing the methods and techniques described herein. The video processing hardware 1006 may be used to implement, in hardware circuitry, some techniques described in the present document.

In some embodiments, the video coding methods may be implemented using an apparatus that is implemented on a hardware platform as described with respect to FIG. 10 .

FIG. 11 shows a flowchart of an example method 1100 for linear model derivations for cross-component prediction in accordance with the disclosed technology. The method 1100 includes, at step 1110, performing a conversion between a current video block and a bitstream representation of the current video block, wherein, during the conversion, a second set of color component values of the current video block are derived from a first set of color component values included in one or more reference frames, wherein the first set of color component values are usable in a linear model of a video coding step.

Some embodiments may be described using the following clause-based format.

1. A method for video processing, comprising:

performing a conversion between a current video block and a bitstream representation of the current video block, wherein, during the conversion, a second set of color component values of the current video block are derived from a first set of color component values included in one or more reference frames, wherein the first set of color component values are usable in a linear model of a video coding step.

2. The method of clause 1, wherein the first set of color component values are interpolated prior to use in the linear model of the video coding step.

3. The method of any one or more of clauses 1-2, wherein a linear combination of the first set of color component values are usable as parameters in the linear model.

4. The method of clause 1, wherein locations of the first set of color component values included in the one or more reference frames are selected based, at least in part, on motion information of the current video block.

5. The method of clause 4, wherein a position of a luma component value in the one or more reference frames is calculated from a position of a corresponding luma component value in the current video block and the motion information of the current video block.

6. The method of clause 5, wherein the position of the corresponding luma component value is a top-left sample, a center sample, or a bottom-right sample in the current video block.

7. The method of clause 6, wherein the motion information of the current video block corresponds to an integer motion vector or a fractional motion vector.

8. The method of clause 7, wherein the fractional motion vector is derived using a fractional luma component value in the one or more reference frames.

9. The method of clause 7, wherein the integer motion vector is derived by rounding towards zero or away from zero.

10. The method of clause 1, wherein locations of the first set of color component values included in the one or more reference frames are pre-defined positions.

11. The method of any one or more of clauses 1-10, wherein a median or an average of the first set of color component values are used to derive the second set of color component values of the current video block.

12. The method of any one or more of clauses 1-11, wherein the one or more reference frames are pre-defined reference frames.

13. The method of clause 12, wherein the pre-defined reference frames include a frame with a reference index of a reference picture list.

14. The method of clause 13, wherein the reference index is zero and the reference picture list is zero.

15. The method of clause 13, wherein the reference index and/or the reference picture list is signaled in the bitstream representation associated with one or more of the following: a sequence, a picture, a tile, a group, a slice, a tile, a coding tree unit row, or a video block.

16. The method of clause 1, wherein the second set of color component values of the current video block are derived from a mathematical mean or a weighted average of the first set of color component values included in the one or more reference frames.

17. The method of clause 1, wherein the second set of color component values of the current video block are selectively derived from the first set of color component values included in the one or more reference frames, based on whether the current video block is a bi-prediction coded block.

18. The method of clause 17, wherein the second set of color component values of the current video block are individually derived for each prediction direction of the first set of color component values.

19. The method of clause 1, wherein the second set of color component values of the current video block are selectively derived from the first set of color component values included in the one or more reference frames, based on whether the current video block is associated with sub-block-based prediction.

20. The method of clause 1, wherein the sub-block-based prediction corresponds to affine prediction or alternative temporal motion vector prediction (ATMVP).

21. The method of any one or more of clauses 19-20, wherein the second set of color component values of the current video block are derived for individual sub-blocks.

22. The method of any one or more of clauses 19-21, wherein the second set of color component values of the current video block are derived for an entirety of the current video block regardless of the sub-block-based prediction.

23. The method of any one or more of clauses 19-22, wherein the first set of color component values included in one or more reference frames are selected based, at least in part on, a motion vector of a sub-block of the current video block.

24. The method of any one or more of clauses 1-23, wherein the first set of color component values included in one or more reference frames are intermediate color component values.

25. The method of any one or more of clauses 1-24, wherein the video coding step precedes another video coding step.

26. The method of clause 25, wherein the first set of color component values included in the one or more reference frames are selected based, at least in part on, an intermediate motion vector of the current video block or a sub-block of the current video block, and wherein the intermediate motion vector is calculated prior to the another video coding step.

27. The method of any one or more of clauses 24-26, wherein the another video coding step includes one or a combination of the following steps: a Bi-Directional Optical Flow (BDOF) step, a decoder-side motion vector refinement (DMVR) step, a prediction refinement optical flow (PROF) step.

28. The method of any one or more of clauses 1-27, wherein the first set of color component values included in the one or more reference frames correspond to M×N luma component values associated with a corresponding luma block.

29. The method of clause 28, wherein the corresponding luma block is a collocated luma block of the current video block.

30. The method of clause 29, wherein a product of M and N is smaller than a product of a block width and a block height of the collocated luma block of the current video block.

31. The method of any one or more of clauses 27-30, wherein the first set of color component values included in the one or more reference frames correspond to at least a portion of reference samples identified at positions of neighboring luma samples of the collocated luma block.

32. The method of any one or more of clauses 1-31, wherein the first set of color component values are down sampled prior to use in the linear model of the video coding step.

33. The method of clause 1, wherein the second set of color component values of the current video block are selected, based, at least in part on, one or more of the following information of the current video block: a quantization parameter, a coding mode, or a picture order count (POC).

34. The method of clause 31, wherein the positions of the neighboring luma samples are such that a top left sample of the collocated luma block is covered.

35. The method of clause 28, wherein the first set of color component values included in the one or more reference frames correspond to at least a portion of reference samples identified at positions external to the corresponding luma block.

36. The method of clause 28, wherein the second set of color component values of the current video block are selectively derived from the first set of color component values included in the one or more reference frames, based on availability of neighboring samples of the corresponding luma block.

37. The method of clause 28, wherein the availability of the neighboring samples of the corresponding luma block is based on one or more of: a use of a coding mode of the current video block, a use of a coding mode of the neighboring samples of the corresponding luma block, a use of a type of a filter associated with the neighboring samples of the corresponding luma block, or a location of the neighboring samples of the corresponding luma block relative to the current video blocks or sub-blocks thereof.

38. The method of clause 28, further comprising:

in response to a lack of the availability of the neighboring samples of the corresponding luma block, substituting, filling, or padding unavailable samples with other samples.

39. The method of clause 28, further comprising:

applying a smoothing filter to samples neighboring the corresponding luma block.

40. A method for video processing, comprising:

performing a conversion between a current video block and a bitstream representation of the current video block, wherein, during the conversion, a second set of color component values of the current video block are derived from a first set of color component values included in one or more reference frames, wherein the first set of color component values are usable in a linear model of a video coding step; and

in response to determining that the first set of color component values included in the one or more reference frames is a collocated luma block of the current video block, selectively enabling or disabling derivation of the second set of color component values of the current video block, based on one or more conditions associated with the collocated luma block of the current video block.

41. The method of clause 40, wherein the one or more conditions associated with the collocated luma block of the current video block include: a partition size of the collocated luma block, a number of coding units of the collocated luma block achieving a threshold number, a top-left luma sample of the collocated luma block achieving a threshold size, a partition tree depth of the collocated luma block, or a corresponding luma block covering the top-left luma sample of the collocated luma block, or a corresponding luma block covering the top-left luma sample of the collocated luma block and additionally included within a bounding box of pre-defined size.

42. The method of clause 40, wherein information indicating the selectively enabling or disabling the derivation is included in the bitstream representation.

43. The method of clause 28, wherein the availability of neighboring samples of the corresponding luma block is associated with checking for the neighboring samples according to a pre-defined order.

44. The method of any one or more of clause 1-43, wherein the second set of color component values of the current video block are stored for use in connection with one or more other video blocks.

45. The method of any one or more of clauses 1-44, wherein the linear model corresponds to a cross-component linear model (CCLM) and the video coding step corresponds to a luma mapping with chroma scaling (LMCS) mode.

46. The method of any one or more of clauses 1-45, wherein the current video block is an inter-coded block, a bi-prediction coded block, or an intra block copy (IBC) coded block.

47. The method of any one or more of clauses 1-46, wherein the first set of color component values correspond to luma sample values and the second set of color component values correspond to chroma scaling factors.

48. An apparatus in a video system comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to implement the method in any one of clauses 1 to 47.

49. A computer program product stored on a non-transitory computer readable media, the computer program product including program code for carrying out the method in any one of clauses 1 to 47.

FIG. 12 is a block diagram showing an example video processing system 1200 in which various techniques disclosed herein may be implemented. Various implementations may include some or all of the components of the system 1200. The system 1200 may include input 1202 for receiving video content. The video content may be received in a raw or uncompressed format, e.g., 8 or 10 bit multi-component pixel values, or may be in a compressed or encoded format. The input 1202 may represent a network interface, a peripheral bus interface, or a storage interface. Examples of network interface include wired interfaces such as Ethernet, passive optical network (PON), etc. and wireless interfaces such as Wi-Fi or cellular interfaces.

The system 1200 may include a coding component 1204 that may implement the various coding or encoding methods described in the present document. The coding component 1204 may reduce the average bitrate of video from the input 1202 to the output of the coding component 1204 to produce a coded representation of the video. The coding techniques are therefore sometimes called video compression or video transcoding techniques. The output of the coding component 1204 may be either stored, or transmitted via a communication connected, as represented by the component 1206. The stored or communicated bitstream (or coded) representation of the video received at the input 1202 may be used by the component 1208 for generating pixel values or displayable video that is sent to a display interface 1210. The process of generating user-viewable video from the bitstream representation is sometimes called video decompression. Furthermore, while certain video processing operations are referred to as “coding” operations or tools, it will be appreciated that the coding tools or operations are used at an encoder and corresponding decoding tools or operations that reverse the results of the coding will be performed by a decoder.

Examples of a peripheral bus interface or a display interface may include universal serial bus (USB) or high definition multimedia interface (HDMI) or Displayport, and so on. Examples of storage interfaces include SATA (serial advanced technology attachment), PCI, IDE interface, and the like. The techniques described in the present document may be embodied in various electronic devices such as mobile phones, laptops, smartphones or other devices that are capable of performing digital data processing and/or video display.

FIG. 13 shows a flowchart of an example method for visual media processing. Steps of this flowchart are discussed in connection with example embodiments 7d and 7e8 in Section 4 of this document. At step 1302, the process performs a conversion between a current chroma video block of visual media data and a bitstream representation of the current chroma video block, wherein, during the conversion, a chroma residual of the current chroma video block is scaled based on a scaling coefficient, wherein the scaling coefficient is derived at least based on luma samples located in predefined positions.

FIG. 14 shows a flowchart of an example method for visual media processing. Steps of this flowchart are discussed in connection with example embodiment 1 in Section 4 of this document. At step 1402, the process performs a conversion between a current video block of visual media data and a bitstream representation of the current video block, wherein, during the conversion, a second set of color component values of the current video block are derived, using a cross-component linear model (CCLM) and/or a luma mapping with chroma scaling (LMCS) mode processing step, from a first set of color component values of the visual media data.

FIG. 15 shows a flowchart of an example method for visual media processing. Steps of this flowchart are discussed in connection with example embodiment 7 in Section 4 of this document. At step 1502, the process performs a conversion between a current video block of visual media data and a bitstream representation of the current video block, wherein, during the conversion, one or more reconstruction samples associated with a current frame of the visual media data are used to derive a chroma residual scaling factor in a luma mapping with chroma scaling (LMCS) mode processing step.

FIG. 16 shows a flowchart of an example method for visual media processing. Steps of this flowchart are discussed in connection with example embodiment 8 in Section 4 of this document. At step 1602, the process performs a conversion between a current video block of visual media data and a bitstream representation of the current video block, wherein, during the conversion, one or more luma prediction samples or luma reconstruction samples in a current frame other than a reference frame and are used to derive a chroma residual scaling factor in a luma mapping with chroma scaling (LMCS) mode processing step.

FIG. 17 shows a flowchart of an example method for visual media processing. Steps of this flowchart are discussed in connection with example embodiments 11a, 11b, 11c, and 11d in Section 4 of this document. At step 1702, the process checks, during a conversion between a current chroma video block and a bitstream representation of the current chroma video block, availability of one or more neighboring luma blocks of a corresponding luma block which covers a top-left sample of a collocated luma block. At step 1704, the process determines, based on the availability of one or more neighboring luma blocks, whether to retrieve neighboring luma samples of the corresponding luma block. At step 1706, the process derives, based on the determining, a scaling coefficient. At step 1708, the process scales, based on the scaling coefficient, a chroma residual of the current chroma video block to generate a scaled chroma residual. At step 1710, the process performs the conversion based on the scaled chroma residual.

FIG. 18 shows a flowchart of an example method for visual media processing. Steps of this flowchart are discussed in connection with example embodiment 11 in Section 4 of this document. At step 1802, the process derives, during a conversion between a current video block of visual media data and a bitstream representation of the current video block, a second set of color component values of the current video block from a first set of color component values of the visual media data using a model associated with a processing step, wherein the first set of color component values are neighboring samples of a corresponding luma block which covers a top-left sample of a collocated luma block.

FIG. 19 shows a flowchart of an example method for visual media processing. Steps of this flowchart are discussed in connection with example embodiment 12 in Section 4 of this document. At step 1902, the process, during a conversion between a current chroma video block of visual media data and a bitstream representation of the current chroma video block, makes a determination of selectively enabling or disabling application of a cross-component linear model (CCLM) and/or a chroma residual scaling (CRS) on the current chroma video block, based at least in part on one or more conditions associated with a collocated luma block of the current chroma video block.

FIG. 20 shows a flowchart of an example method for visual media encoding. Steps of this flowchart are discussed in connection with example embodiment 14 in Section 4 of this document. At step 2002, the process selectively enables or disables application of a luma dependent chroma residual scaling (CRS) on chroma components of a current video block of visual media data for encoding the current video block in a video region of a visual media data into a bitstream representation of the visual media data. At step 2004, the process makes a determination of including or excluding a field in the bitstream representation of the visual media data, wherein the field is indicative of the selectively enabling or disabling and, if included, is signaled, other than at a first syntax level associated with the current video block.

FIG. 21 shows a flowchart of an example method for visual media decoding. Steps of this flowchart are discussed in connection with example embodiment 14 in Section 4 of this document. At step 2102, the process parses a field in a bitstream representation of visual media data, wherein the field is included in a level other than at a first syntax level associated with a current video block. At step 2104, the process selectively enables or disables, based on the field, application of a luma dependent chroma residual scaling (CRS) on chroma components of the current video block of visual media data for generating a decoded video region from the bitstream representation.

FIG. 22 shows a flowchart of an example method for visual media encoding. Steps of this flowchart are discussed in connection with example embodiment 15 in Section 4 of this document. At step 2202, the process selectively enables or disables application of a cross-component linear model (CCLM) on a current video block of visual media data for encoding the current video block into a bitstream representation of the visual media data. At step 2204, the process makes a determination of including or excluding a field in a bitstream representation of the visual media data, wherein the field is indicative of the selectively enabling or disabling and, if included, is signaled, other than at a first syntax level associated with the current video block.

FIG. 23 shows a flowchart of an example method for visual media decoding. Steps of this flowchart are discussed in connection with example embodiment 15 in Section 4 of this document. At step 2302, the process parses a field in a bitstream representation of visual media data, wherein the field is included a level other than at a first syntax level associated with a current video block. At step 2304, the process selectively enables or disables, based on the field, application of a cross-component linear model (CCLM) on the current video block of visual media data for generating a decoded video region from the bitstream representation.

Some embodiments discussed in the present document are now presented in clause-based format.

X1. A method for visual media processing, comprising:

performing a conversion between a current chroma video block of visual media data and a bitstream representation of the current chroma video block, wherein, during the conversion, a chroma residual of the current chroma video block is scaled based on a scaling coefficient, wherein the scaling coefficient is derived at least based on luma samples located in predefined positions.

X2. The method of clause X1, wherein the scaling coefficient is computed using a function applied on the luma samples in the predefined positions.

X3. The method of clause X2, wherein the function is a median value function or a rounding-based average value function.

X4. The method of clause X1, wherein the predefined positions are determined based on a collocated luma block corresponding to the current chroma video block.

A1. A method for visual media processing, comprising:

performing a conversion between a current video block of visual media data and a bitstream representation of the current video block, wherein, during the conversion, a second set of color component values of the current video block are derived, using a cross-component linear model (CCLM) and/or a luma mapping with chroma scaling (LMCS) mode processing step, from a first set of color component values of the visual media data.

A2. The method of clause A1, wherein the first set of color component values are reference samples of the current video block and the second set of color components values is a chroma residual scaling factor in the LMCS mode.

A3. The method of clause A2, wherein the reference samples are reference luma samples that are interpolated prior to deriving a chroma residual scaling factor.

A4. The method of clause A1, wherein the first set of color component values are reference samples included in different reference frames.

A5. The method of any one or more of clauses A1-A4, wherein a position of a reference sample is calculated from a position of a corresponding luma component value in the current video block and a motion information of the current video block.

A6. The method of clause A5, wherein the position of the corresponding luma component value is a top-left sample, a center sample, or a bottom-right sample in the current video block.

A7. The method of clause A6, wherein the motion information of the current video block corresponds to an integer motion vector or a fractional motion vector.

A8. The method of clause A7, wherein the fractional motion vector is derived using a fractional luma component value in a reference frame.

A9. The method of clause A7, wherein the integer motion vector is derived by rounding towards zero or away from zero.

A10. The method of any one or more of clauses A1-A2, wherein the first set of color component values are included in pre-defined reference frames of the visual media data.

A11. The method of any one or more of clauses A1-A10, wherein a median or an average of the first set of color component values are used to derive the second set of color component values of the current video block.

A12. The method of clause A10, wherein the pre-defined reference frames include a frame with a reference index of a reference picture list.

A13. The method of clause A12, wherein the reference index is zero and the reference picture list is zero.

A14. The method of any one or more of clauses A1-A2, wherein the first set of color component values are included in multiple reference frames of the visual media data and a weighted combination of the first set of color component values are used to derive the second set of color component values.

A15. The method of clause A13, wherein the reference index and/or the reference picture list is/are signaled as fields in the bitstream representation associated with one or more of the following: a sequence, a group of pictures, a picture, a tile, a tile group, a slice, a subpicture, a coding tree unit row, a coding tree unit, a virtual pipeline data unit (VPDU), or a video block.

A16. The method of clause A1, wherein the second set of color component values of the current video block are selectively derived from the first set of color component values, based on whether the current video block is a bi-prediction coded block.

A17. The method of clause A16, wherein the second set of color component values of the current video block are individually derived for each prediction direction associated with the first set of color component values.

A18. The method of clause A1, wherein the second set of color component values of the current video block are selectively derived from the first set of color component values, based on whether the current video block is associated with sub-block-based prediction.

A19. The method of clause A18, wherein the sub-block-based prediction corresponds to affine prediction or alternative temporal motion vector prediction (ATMVP).

A20. The method of any one or more of clauses A18-A19, wherein the second set of color component values of the current video block are derived for individual sub-blocks.

A21. The method of any one or more of clauses A18-A19, wherein the second set of color component values of the current video block are derived for an entirety of the current video block regardless of the sub-block-based prediction.

A22. The method of any one or more of clauses A18-A21, wherein the first set of color component values are selected based, at least in part on, a motion vector of a sub-block of the current video block.

A23. The method of any one or more of clauses A18-A21, wherein a motion vector associated with a sub-block of the current video block are used to select the first set of color component values.

A24. The method of any one or more of clauses A1-A23, wherein the first set of color component values are intermediate color component values.

A25. The method of any one or more of clauses A1-A24, wherein the LMCS mode processing step precedes another subsequent processing step.

A26. The method of clause A25, wherein the first set of color component values are selected based, at least in part on, an intermediate motion vector of the current video block or a sub-block of the current video block, and wherein the intermediate motion vector is calculated prior to the another video coding step.

A27. The method of any clause A26, wherein the another processing step includes one or a combination of the following: a Bi-Directional Optical Flow (BDOF) step, a decoder-side motion vector refinement (DMVR) step, or a prediction refinement optical flow (PROF) step.

A28. A method for visual media processing, comprising:

performing a conversion between a current video block of visual media data and a bitstream representation of the current video block, wherein, during the conversion, one or more reconstruction samples associated with a current frame of the visual media data are used to derive a chroma residual scaling factor in a luma mapping with chroma scaling (LMCS) mode processing step.

A29. The method of clause A28, wherein the current video block is intra block copy (IBC) coded.

A30. The method of clause A28, wherein the one or more reconstruction samples are reference samples in a reference block associated with the current frame.

A31. The method of clause A28, wherein locations of the one or more reconstruction samples are pre-defined.

A32. The method of clause A31, wherein the one or more reconstruction samples are reconstructed luma samples located at an above row and a left column that are adjacent to a block covering a corresponding luma block of the current video block.

A33. The method of clause A28, wherein locations of the one or more reconstruction samples are based on a position of a corresponding luma block of the current video block and a motion information of the current video block.

A34. A method for visual media processing, comprising:

performing a conversion between a current video block of visual media data and a bitstream representation of the current video block, wherein, during the conversion, one or more luma prediction samples or luma reconstruction samples in a current frame other than a reference frame and are used to derive a chroma residual scaling factor in a luma mapping with chroma scaling (LMCS) mode processing step.

A35. The method of any clause A34, wherein the one or more luma prediction samples or luma reconstruction samples are located in a neighboring region of a M×N luma block that covers a corresponding luma block.

A36. The method of clause A35, wherein the corresponding luma block is a collocated luma block of the current video block.

A37. The method of clause A36, wherein a product of M and N is smaller than a product of a block width and a block height of the collocated luma block of the current video block.

A38. The method of clause A36, wherein M and N are a predefined width and a predefined height of a video block that covers the collocated luma block of the current video block.

A39. The method of clause A36, wherein M and N are a width and a height of a virtual pipeline data unit (VPDU) that covers the collocated luma block of the current video block.

A40. The method of any one or more of clauses A1-A39, wherein, during the conversion, the reference samples are directly used or down sampled prior to use in derivation.

A41. The method of any or more of clauses A1-A40, wherein samples used in deriving chroma residual scaling factors are selected, based, at least in part on, one or more of the following information of the current video block: a quantization parameter, a coding mode, or a picture order count (POC).

A42. The method of any one or more of clauses A1-A41, wherein the current video block is an inter-coded block, an intra-coded block, a bi-prediction coded block, or an intra block copy (IBC) coded block.

A43. The method of any one or more of clauses A1-A42, wherein the first set of color component values correspond to luma sample values and the second set of color component values correspond to chroma scaling factors of the current video block.

A44. The method of any one or more of clauses A1-A43, wherein the conversion includes generating the bitstream representation from the current video block.

A45. The method of any one or more of clauses A1-A43, wherein the conversion includes generating pixel values of the current video block from the bitstream representation.

A46. A video encoder apparatus comprising a processor configured to implement a method recited in any one or more of clauses A1-A45.

A47. A video decoder apparatus comprising a processor configured to implement a method recited in any one or more of clauses A1-A45.

A48. A computer readable medium having code stored thereon, the code embodying processor-executable instructions for implementing a method recited in any one or more of clauses A1-A38.

Y1. A method for visual media processing, comprising:

checking, during a conversion between a current chroma video block and a bitstream representation of the current chroma video block, availability of one or more neighboring luma blocks of a corresponding luma block which covers a top-left sample of a collocated luma block;

determining, based on the availability of one or more neighboring luma blocks, whether to retrieve neighboring luma samples of the corresponding luma block;

deriving, based on the determining, a scaling coefficient;

scaling, based on the scaling coefficient, a chroma residual of the current chroma video block to generate a scaled chroma residual; and

performing the conversion based on the scaled chroma residual.

Y2. The method of clause Y1, wherein the one or more neighboring luma blocks include a left neighboring luma block and an above neighboring luma block.

Y3. The method of clause Y1, wherein the neighboring luma samples includes one or more left neighboring sample columns and/or one or more above neighboring sample rows of the corresponding luma block.

Y4. The method of any one or more of clauses Y1 or Y3, wherein the neighboring luma samples are retrieved and rounding-based averaged to derive the scaling coefficient in a case that the one or more neighboring luma blocks are available.

Y5. The method of clause Y1 or Y3, wherein the neighboring luma samples are retrieved and a median value of the neighboring luma samples is used to derive the scaling coefficient in a case that the one or more neighboring luma blocks are available.

Y6. The method of clause Y3, wherein a number of neighboring luma samples is N, wherein 1<=N<=2W+2H, W and H are a width and a height of the corresponding luma block.

Y7. The method of clause Y1 or Y2, wherein availability of the one or more neighboring luma blocks is determined based on a width and/or a height of a current picture, a tile, a tile group, a virtual pipeline data unit (VPDU), or a slice.

Y8. The method of clause Y7, wherein the one or more neighboring luma blocks are unavailable in a case that the one or more neighboring block are located in a different picture, a different tile, a different tile group, a different VPDU, or a different slice.

Y9. The method of clause Y1, wherein the neighboring luma samples of the corresponding are skipped from retrieval in a case that the one or more neighboring luma blocks are unavailable.

Y10. The method of clause Y9, wherein the scaling coefficient is derived in a case that the neighboring luma samples of the corresponding are skipped from retrieval.

Y11. The method of clause Y10, wherein the scaling coefficient is derived base on a default value.

Y12. The method of clause Y11, wherein the default value is based on a bit depth of the current chroma video block and the collocated luma block.

Y13. The method of clause Y12, wherein the default value is expressed as 1<<(bitDepth−1), where bitDepth denotes the bit depth of the current chroma video block and the collocated luma block.

Y14. The method of clause Y1, wherein the neighboring luma samples are reconstructed based on forward mapping.

B1. A method for visual media processing, comprising:

deriving, during a conversion between a current video block of visual media data and a bitstream representation of the current video block, a second set of color component values of the current video block from a first set of color component values of the visual media data using a model associated with a processing step, wherein the first set of color component values are neighboring samples of a corresponding luma block which covers a top-left sample of a collocated luma block.

B2. The method of clause B1, wherein the current video block is one of: an intra-coded video block having a dual tree partition or an intra-coded block having a single tree partition or an inter-coded video block having a single tree partition.

B3. The method of any one or more of clauses B1-B2, wherein the first set of color component values correspond to at least a portion of reference samples identified at positions external to the corresponding luma block.

B4. The method of clause B3, wherein the portion of reference samples identified at the positions external to the corresponding luma block include samples adjacent to the corresponding luma coding block.

B5. The method of clause B4, wherein the samples adjacent to the corresponding luma coding block include N samples located at left neighboring columns and/or above neighboring rows of the corresponding luma coding block, where N=1 . . . 2W+2H, W and H are a width and a height of the corresponding luma coding block.

B6. The method of clause B5, wherein an above neighboring luma sample is located at (xCb+W/2, yCb−1) or (xCb−1, yCb−1) when the top-left sample of the collocated luma block is located at (xCb, yCb).

B7. The method of clause B5, wherein a left neighboring luma sample is located at (xCb+W−1, yCb−1) when the top-left sample of the collocated luma block is located at (xCb, yCb).

B8. The method of clause B4, wherein the portion of reference samples identified at the positions external to the corresponding luma block are at pre-defined positions.

B9. The method of clause B4, wherein the second set of color component values of the current video block are derived based on a median or arithmetic mean of N samples located at left neighboring columns and/or above neighboring rows of the corresponding luma coding block.

B10. The method of any one or more of clauses B1-B2, wherein the second set of color component values of the current video block are selectively derived from the first set of color component values, based on availability of neighboring samples of the corresponding luma block.

B11. The method of clause B10, wherein the availability of the neighboring samples of the corresponding luma block is based on one or more of: a use of a coding mode of the current video block, a use of a coding mode of the neighboring samples of the corresponding luma block, a use of a type of a filter associated with the neighboring samples of the corresponding luma block, a location of the neighboring samples of the corresponding luma block relative to the current video blocks or sub-blocks thereof, a width of a current picture/subpicture/tile/tile group/VPDU/slice, and/or a height of a current picture/subpicture/tile/tile group/VPDU/slice/coding tree unit (CTU) row.

B12. The method of clause B10, further comprising:

in response to determining a lack of the availability of the neighboring samples of the corresponding luma block, substituting, filling, or padding unavailable samples with other samples.

B13. The method of clause B12, wherein the lack of the availability of the neighboring samples of the corresponding luma block is based at least in part on determining that when a coding mode of the current video block is inter mode, the coding mode of the neighbouring samples are intra mode and/or an intra block copy (IBC) mode and/or a combined inter-intra prediction (CIIP) mode and/or a localized illuminate compensation (LIC) mode.

B14. The method of clause B12, wherein the lack of the availability of the neighboring samples of the corresponding luma block is based at least in part on determining that when a coding mode of the current video block is inter mode, the neighbouring samples are subjected to a diffusion filter and/or a bilateral filter and/or a Hadamard transform filter.

B15. The method of clause B12, wherein the lack of the availability of the neighboring samples of the corresponding luma block is based at least in part on determining that a neighboring block is located outside a current picture/subpicture/tile/tile group/VPDU/slice/coding tree unit (CTU) row associated with the current video block.

B16. The method of any one or more of clauses B12-B15, further comprising:

in response to determining the lack of the availability of the neighboring samples of the corresponding luma block, disabling derivation of the second set of color component values of the current video block from the first set of color component values.

B17. The method of clause B10, further comprising:

in response to determining that a number of available neighboring samples is less than a threshold, disabling derivation of the second set of color component values of the current video block from the first set of color component values.

B18. The method of clause B17, wherein the threshold is one.

B19. The method of clause B12, wherein, if a neighboring sample is determined to be not available, then the neighboring sample is filled by 1<<(bitDepth−1) samples, where bitDepth denotes a bit depth of the first set of samples of color component values or the second set of samples of color component values.

B20. The method of clause B12, wherein, if a neighboring sample is determined to be not available, then the neighboring sample is substituted by a first available adjacent sample in accordance with a pre-defined checking order.

B21. The method of clause B13, wherein, if a neighboring sample is determined to be not available, then the neighboring sample is padded using one or more of: a left neighbouring sample, a right neighbouring sample, a top neighbouring sample, or a bottom neighbouring sample.

B22. The method of clause B10, further comprising:

applying a smoothing filter to samples neighboring the corresponding luma block that are used in derivation of the second set of color component values of the current video block.

B23. The method of clause, B22, wherein the smoothing filter includes one or more of the following: a bilateral filter, a Hadamard transform-based filter, or a forward mapping of reshaper domain.

B24. The method of any one or more of clauses B1-B23, wherein the second set of color component values of the current video block are stored for use in connection with one or more other video blocks.

B25. The method of any one or more of clauses B1-B23, wherein the model corresponds to a cross-component linear model (CCLM) and/or the processing step corresponds to a luma mapping with chroma scaling (LMCS) mode.

B26. The method of any one or more of clauses B1-B23, wherein the current video block is an intra-coded block, an inter-coded block, a bi-prediction coded block, or an intra block copy (IBC) coded block.

B27. The method of any one or more of clauses B1-B23, wherein the first set of color component values correspond to luma sample values and the second set of color component values correspond to chroma scaling factors of the current video block.

B28. The method of any one or more of clauses B1-B23, wherein the conversion includes generating the bitstream representation from the current video block.

B29. The method of any one or more of clauses B1-B23, wherein the conversion includes generating pixel values of the current video block from the bitstream representation.

B30. A video encoder apparatus comprising a processor configured to implement a method recited in any one or more of clauses B1-B23.

B31. A video decoder apparatus comprising a processor configured to implement a method recited in any one or more of clauses B1-B23.

B32. A computer readable medium having code stored thereon, the code embodying processor-executable instructions for implementing a method recited in any one or more of clauses B1-B23.

C1. A method for visual media processing, comprising:

during a conversion between a current chroma video block of visual media data and a bitstream representation of the current chroma video block, making a determination of selectively enabling or disabling application of a cross-component linear model (CCLM) and/or a chroma residual scaling (CRS) on the current chroma video block, based at least in part on one or more conditions associated with a collocated luma block of the current chroma video block.

C2. The method of clause C1, wherein the one or more conditions associated with the collocated luma block of the current chroma video block include: a partition size of the collocated luma block, a number of coding units of the collocated luma block achieving a threshold number, a top-left luma sample of the collocated luma block achieving a threshold size, a partition tree depth of the collocated luma block, or a corresponding luma block covering the top-left luma sample of the collocated luma block, a corresponding luma block covering the top-left luma sample of the collocated luma block and additionally included within a bounding box of pre-defined size, a coding mode of one or multiple coding units (CUs) covering at least one sample of the collocated luma block, and/or a dimension of the current chroma video block.

C3. The method of any one or more of clauses C1-C2, wherein the application of the CCLM and/or the CRS on the current chroma video block is disabled in response to determining that the collocated luma block of the current chroma video block is divided into multiple partitions.

C4. The method of any one or more of clauses C1-C2, wherein the application of the CCLM and/or the CRS on the current chroma video block is enabled in response to determining that the collocated luma block of the current chroma video block is not divided into multiple partitions.

C5. The method of any one or more of clauses C1-C2, wherein the application of the CCLM and/or the CRS on the current chroma video block is disabled in response to determining that the collocated luma block of the current chroma video block includes more than one of: a threshold number of coding units and/or a threshold number of partition units and/or a threshold number of transform units.

C6. The method of clause C5, wherein the threshold number is one.

C7. The method of clause C5, wherein the threshold number is based at least in part on whether the CCLM and/or the CRS is applied.

C8. The method of clause C5, wherein the threshold number is fixed or included in the bitstream representation.

C9. The method of clause C5, wherein the threshold number is based at least in part on profiles/levels/tiers associated with the current chroma video block.

C10. The method of clause C5, wherein the coding units and/or the partition units and/or the transform units are fully located within the collocated luma block.

C11. The method of clause C5, wherein the coding units and/or the partition units and/or the transform units are partially located within the collocated luma block.

C12. The method of clause C11, wherein the coding units and/or the partition units and/or the transform units are partially located along a boundary of the collocated luma block.

C13. The method of clause C5, wherein coding units and/or the partition units and/or the transform units are associated with sub-block-based prediction.

C14. The method of clause C13, wherein the sub-block-based prediction corresponds to Intra Sub-Partitions (ISP) or affine prediction or alternative temporal motion vector prediction (ATMVP).

C15. The method of any one or more of clauses C1-C2, wherein the application of the CCLM and/or the CRS on the current chroma video block is disabled in response to determining that a coding unit and/or a partition unit and/or a transform unit covering a top-left luma sample of the collocated luma block is larger than a pre-defined block size.

C16. The method of clause C15, wherein the collocated luma block is of size 32×32 and included within a corresponding luma block of size 64×64, and the pre-defined luma block size is 32×64.

C17. The method of clause C2, wherein the application of the CCLM and/or the CRS on the current chroma video block is enabled in response to determining that the collocated luma block of the current chroma video block is not split, and the corresponding luma block covering the top-left luma sample of the collocated luma block is entirely included within the bounding box of pre-defined size.

C18. The method of clause C17, wherein the corresponding luma block is of size 32×64 and the bounding box is of size 40×70.

C19. The method of clause C17, wherein the pre-defined size of the bounding box is based in part on a size of a coding tree unit (CTU) associated with the current chroma video block and/or a size of a coding unit (CU) associated with the current chroma video block.

C20. The method of any one or more of clauses C1-C2, wherein the collocated luma block of the current chroma video block is divided into multiple partitions and prediction samples or reconstructed samples inside one or more of the multiple partitions are used to derive values associated with the CRS of the current chroma video block.

C21. The method of clause C20, wherein an average of the prediction samples or the reconstructed samples inside a first partition of the collocated luma block of the current chroma video block is used to derive the values associated with the CRS of the current chroma video block.

C22. The method of clause C20, wherein a top-left prediction sample or a top-left reconstructed sample inside a first partition of the collocated luma block of the current chroma video block is used to derive the values associated with the CRS of the current chroma video block.

C23. The method of clause C20, wherein a center prediction sample or a center reconstructed sample inside a first partition of the collocated luma block of the current chroma video block is used to derive the color component values of the current chroma video block.

C24. The method of clause C2, wherein the application of the CCLM and/or the CRS on the current chroma video block is disabled in response to determining that the coding mode of the one or multiple coding units (CUs) covering the at least one sample of the collocated luma block is one of: an affine mode, a bi-prediction mode, a Bi-Directional Optical Flow (BDOF) mode, a DMVR mode, a matrix affine prediction mode, an inter mode, or an Intra Sub-Partitions (ISP) mode.

C25. The method of clause C2, wherein the one or multiple coding units (CUs) covering the at least one sample of the collocated luma block is the corresponding luma block.

C26. The method of any one or more of clauses C1-C25, further comprising:

indicating, based on a field in the bitstream representation, that the CCLM and/or the CRS is selectively enabled or disabled on the current chroma video block.

C27. The method of any one or more of clauses C1-C26, wherein the selectively enabling or disabling the application of the CCLM and/or the CRS on the current chroma video block is performed on one or more sub-blocks of the current chroma video block.

C28. The method of clause C27, wherein the one or more sub-blocks of the current chroma video block is of size 2×2 or 4×4.

C29. The method of clause C27, wherein the application of the CCLM and/or the CRS is enabled for a sub-block of the current chroma video block when the corresponding luma coding block of the current chroma video block covers all samples of a corresponding block of the subblock.

C30. The method of clause C27, wherein the application of the CCLM and/or the CRS is disabled for a sub-block of the current chroma video block when all samples of a corresponding block of the subblock are not covered by the corresponding luma coding block.

C31. The method of clause C27, wherein parameters of the CCLM and/or the CRS are associated with each sub-block of the current chroma video block.

C32. The method of clause C27, wherein the selectively enabling or disabling the application of the CCLM and/or the CRS on a sub-block of the current chroma video block is based on samples included within the collocated luma block.

C33. The method of any one or more of clauses C1-C32, wherein the current chroma video block is an inter-coded block, an intra-coded block, a bi-prediction coded block, or an intra block copy (IBC) coded block.

C34. The method of any one or more of clauses C1-C33, wherein the conversion includes generating the bitstream representation from the current chroma video block.

C35. The method of any one or more of clauses C1-C33, wherein the conversion includes generating pixel values of the current chroma video block from the bitstream representation.

C36. A video encoder apparatus comprising a processor configured to implement a method recited in any one or more of clauses C1-C33.

C37. A video decoder apparatus comprising a processor configured to implement a method recited in any one or more of clauses C1-C33.

C38. A computer readable medium having code stored thereon, the code embodying processor-executable instructions for implementing a method recited in any one or more of clauses C1-C33.

D1. A method for visual media encoding, comprising:

selectively enabling or disabling application of a luma dependent chroma residual scaling (CRS) on chroma components of a current video block of visual media data for encoding the current video block in a video region of a visual media data into a bitstream representation of the visual media data; and

making a determination of including or excluding a field in the bitstream representation of the visual media data, wherein the field is indicative of the selectively enabling or disabling and, if included, is signaled, other than at a first syntax level associated with the current video block.

D2. A method for visual media decoding, comprising:

parsing a field in a bitstream representation of visual media data, wherein the field is included in a level other than at a first syntax level associated with a current video block; and

selectively enabling or disabling, based on the field, application of a luma dependent chroma residual scaling (CRS) on chroma components of the current video block of visual media data for generating a decoded video region from the bitstream representation.

D3. The method of any one or more of clauses D1-D2, wherein the first syntax level is a tile group header level, and wherein the field is included at one of: a sequence parameter set (SPS) associated with the current video block, a tile associated with the current video block, a coding tree unit (CTU) row associated with the current video block, a coding tree unit (CTU) associated with the current video block, a virtual pipeline data unit (VPDU) associated with the current video block, or a coding unit (CU) associated with the current video block.

D4. The method of any one or more of clauses D1-D3, wherein the field is a flag denoted as chroma_residual_scale_flag.

D5. The method of any one or more of clauses D1-D4, wherein the field is associated with a syntax level, and wherein, if the field is one, the application of the luma dependent chroma residual scaling below the syntax level is enabled, and if the field is zero, the application of the luma dependent chroma residual scaling below the syntax level is disabled.

D6. The method of clause D5, wherein the field is associated with a partition node level, and wherein, if the field is one, the application of the luma dependent chroma residual scaling below the partition node level is enabled, and if the field is zero, the application of the luma dependent chroma residual scaling below the partition node level is disabled.

D7. The method of any one or more of clauses D1-D4, wherein the field is associated with a threshold dimension, and wherein, if the field is one, the application of the luma dependent chroma residual scaling for video blocks at or above the threshold dimension is enabled, and if the field is zero, the application of the luma dependent chroma residual scaling for video blocks below the threshold dimension is disabled.

D8. The method of clause D7, wherein the threshold dimension is 32×32.

D9. The method of any one or more of clauses D1-D8, wherein the field is not signaled in the bitstream representation and absence of the field in the bitstream representation is used to infer that the application of the luma dependent chroma residual scaling is disabled, and the field is inferred to be zero.

D10. The method of any one or more of clauses D1-D9, wherein values associated with the luma dependent CRS of the current video block are stored for use in connection with one or more other video blocks.

D11. The method of clause D10, wherein the values associated with the luma dependent CRS are derived subsequent to an encoding or decoding of a luma block.

D12. The method of clause D11, wherein, in the luma block, prediction samples and/or intermediate prediction samples and/or reconstructed samples and/or reconstructed samples before loop filtering are used to derive the values associated with the luma dependent CRS.

D13. The method of clause D12, wherein the loop filtering includes a use of: a deblocking filter and/or a sample adaptive offset (SAO) filter and/or a bilateral filter and/or a Hadamard transform filter and/or an adaptive loop filter (ALF).

D14. The method of clause D11, wherein samples in a bottom row and/or a right column of the luma block are used to derive the values associated with the luma dependent CRS.

D15. The method of clause D11, wherein samples associated with neighboring blocks are used to derive the values associated with the luma dependent CRS.

D16. The method of clause D15, wherein the current video block is an intra-coded block, an inter-coded block, a bi-prediction coded block, or an intra block copy (IBC) coded block.

D17. The method of clause D15, wherein availability of samples associated with the neighboring blocks is checked according to a pre-defined order.

D18. The method of clause D17, wherein the pre-defined order relative to the current video block is one of: left to right, above left to above right, left to above, above left to above right, above to left, above right to above left.

D19. The method of clause D17, wherein the pre-defined order relative to the current video block is one of: below left to left to above right to above to above left.

D20. The method of clause D17, wherein the pre-defined order relative to the current video block is one of: left to above to above right to below left to above left.

D21. The method of clause D17, wherein the pre-defined order is associated with samples in a first-available subset of the neighboring blocks.

D22. The method of clause D15, wherein, if the current video block is an inter-coded block and a neighboring block is an intra-coded block, an IBC-coded block, or a CIIP-coded block, then the samples associated with the neighboring block are determined to be unavailable.

D23. The method of clause D15, wherein, if the current video block is an CIIP-coded block and a neighboring block is an intra-coded block, an IBC-coded block, or a CIIP-coded block, then the samples associated with the neighboring block are determined to be unavailable.

D24. The method of clause D15, further comprising:

in response to determining that a number of the neighboring blocks is less than a threshold value, disabling derivation of the luma dependent CRS.

D25. The method of clause D24, wherein the threshold value is one.

D26. The method of clause D24, wherein, if a sample from a neighboring block is determined to be not available, then the sample is filled by 1<<(bitDepth−1) samples, where bitDepth denotes a bit depth of the chroma components or luma components.

E1. A method for visual media encoding, comprising:

selectively enabling or disabling application of a cross-component linear model (CCLM) on a current video block of visual media data for encoding the current video block into a bitstream representation of the visual media data; and

making a determination of including or excluding a field in a bitstream representation of the visual media data, wherein the field is indicative of the selectively enabling or disabling and, if included, is signaled, other than at a first syntax level associated with the current video block.

E2. A method for visual media decoding, comprising:

parsing a field in a bitstream representation of visual media data, wherein the field is included a level other than at a first syntax level associated with a current video block; and

selectively enabling or disabling, based on the field, application of a cross-component linear model (CCLM) on the current video block of visual media data for generating a decoded video region from the bitstream representation.

E3. The method of any one or more of clauses E1-E2, wherein the first syntax level is a sequence parameter set (SPS) level, and wherein the field is included at one of: a picture parameter set (PPS) associated with the current video block, a slice associated with the current video block, a picture header associated with the current video block, a tile associated with the current video block, a tile group associated with the current video block, a coding tree unit (CTU) row associated with the current video block, a coding tree unit (CTU) associated with the current video block, a virtual pipeline data unit (VPDU) associated with the current video block, or a coding unit (CU) associated with the current video block.

E4. The method of any one or more of clauses E1-E3, wherein the field is a flag denoted as cclm_flag.

E5. The method of clause any one or more of clauses E1-E4, wherein, absence of the field in the bitstream representation is used to infer that the application of the CCLM is disabled.

E6. The method of clause any one or more of clauses E1-E4, wherein, presence of the field in the bitstream representation is used to infer that the application of the CCLM is enabled.

E7. The method of clause E5, wherein, if a dimension of the current video block is less than or equal to a threshold dimension, the field is excluded in the bitstream representation, and thereby exclusion of the field is used to infer that the application of the CCLM is disabled.

E8. The method of clause E7, wherein the threshold dimension is 8×8.

F1. A video encoder apparatus comprising a processor configured to implement a method recited in any one or more of clauses X1-E8.

F2. A video decoder apparatus comprising a processor configured to implement a method recited in any one or more of clauses X1-E8.

F3. A computer readable medium having code stored thereon, the code embodying processor-executable instructions for implementing a method recited in any one or more of clauses X1-E8.

In the present document, the term “video processing” or “visual media processing” may refer to video encoding, video decoding, video compression or video decompression. For example, video compression algorithms may be applied during conversion from pixel representation of a video to a corresponding bitstream representation or vice versa. The bitstream representation of a current video block may, for example, correspond to bits that are either co-located or spread in different places within the bitstream, as is defined by the syntax. For example, a macroblock may be encoded in terms of transformed and coded error residual values and also using bits in headers and other fields in the bitstream. Furthermore, during conversion, a decoder may parse a bitstream with the knowledge that some fields may be present, or absent, based on the determination, as is described in the above solutions. Similarly, an encoder may determine that certain syntax fields are or are not to be included and generate the coded representation accordingly by including or excluding the syntax fields from the coded representation.

From the foregoing, it will be appreciated that specific embodiments of the presently disclosed technology have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention. Accordingly, the presently disclosed technology is not limited except as by the appended claims.

Implementations of the subject matter and the functional operations described in this patent document can be implemented in various systems, digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Implementations of the subject matter described in this specification can be implemented as one or more computer program products, i.e., one or more modules of computer program instructions encoded on a tangible and non-transitory computer readable medium for execution by, or to control the operation of, data processing apparatus. The computer readable medium can be a machine-readable storage device, a machine-readable storage substrate, a memory device, a composition of matter effecting a machine-readable propagated signal, or a combination of one or more of them. The term “data processing unit” or “data processing apparatus” encompasses all apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them.

A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network.

The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).

Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Computer readable media suitable for storing computer program instructions and data include all forms of nonvolatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry.

It is intended that the specification, together with the drawings, be considered exemplary only, where exemplary means an example. As used herein, the use of “or” is intended to include “and/or”, unless the context clearly indicates otherwise.

While this patent document contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described in this patent document should not be understood as requiring such separation in all embodiments.

Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document. 

What is claimed is:
 1. A method of processing video data, comprising: determining, during a conversion between a current chroma video block of a video and a bitstream of the video, that a scaling process is applied on chroma residual samples of the current chroma video block; and performing the conversion by applying the scaling process on the chroma residual samples, wherein in the scaling process, the chroma residual samples are scaled based on at least one scaling factor before being used to reconstruct the current chroma video block, and wherein the at least one scaling factor is derived based on an averaged luma variable computed based on neighboring luma samples of a video unit of the video which is determined based on a luma sample corresponding to a top-left sample of the current chroma video block, and wherein the at least one scaling factor is derived by: checking an availability of each of one or more neighboring luma blocks of the video unit, wherein the each of the one or more neighboring luma blocks comprise at least one sample of the neighboring luma samples, determining, based on the availability of the each of the one or more neighboring luma blocks, whether to retrieve the neighboring luma samples of the video unit, deriving the at least one scaling factor based on the averaged luma variable computed using the neighboring luma samples by a rounding-based average operation in response to the neighboring luma samples being available, and deriving the at least one scaling factor as 1<<(bitDepth−1) in response to the neighboring luma samples being unavailable, where bitDepth is a bit depth of the video, and wherein a size information of a virtual pipeline data unit is further used to derive the at least one scaling factor.
 2. The method of claim 1, wherein the neighboring luma samples are located in predefined positions neighboring to the video unit.
 3. The method of claim 2, wherein the neighboring luma samples located in the predefined positions neighboring to the video unit include reconstructed luma samples external to the video unit.
 4. The method of claim 2, wherein the neighboring luma samples located in the predefined positions neighboring to the video unit include reconstructed luma samples adjacent to the video unit.
 5. The method of claim 4, wherein the reconstructed luma samples adjacent to the video unit include at least one of one or more left neighboring luma sample columns or one or more above neighboring luma sample rows of the video unit.
 6. The method of claim 1, wherein a total number of the neighboring luma samples of the video unit is N, where N is an integer greater than 1, and a range of N depends on a size information of the video unit.
 7. The method of claim 1, wherein the checking the availability of each of the one or more neighboring luma blocks comprises: checking the availability of each of the one or more neighboring luma blocks based on at least one of a width and a height of a video region of the video.
 8. The method of claim 7, wherein in response to a top-left sample of the one neighboring luma block being located outside the video region, the neighboring luma block is treated as unavailable.
 9. The method of claim 1, wherein, for a luma video block of a video region of the video, at least one of the following is performed: 1) a forward mapping process for the luma video block, in which prediction samples of the luma video block are converted from an original domain to a reshaped domain; or 2) an inverse mapping process, which is an inverse operation of the forward mapping process, in which reconstructed samples of the luma video block in the reshaped domain are converted to the original domain, and wherein the neighboring luma samples include reconstructed samples in the reshaped domain.
 10. The method of claim 7, wherein the video region is a picture.
 11. The method of claim 1, wherein the scaling process is based on a piecewise linear model, and wherein an index identifying a piece to which the averaged luma variable belongs, and the at least one scaling factor is derived based on the index.
 12. The method of claim 1, wherein the conversion includes encoding the current chroma video block into the bitstream.
 13. The method of claim 1, wherein the conversion includes decoding the current chroma video block from the bitstream.
 14. An apparatus for processing video data comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to: determine, during a conversion between a current chroma video block of a video and a bitstream of the video, that a scaling process is applied on chroma residual samples of the current chroma video block; and perform the conversion by applying the scaling process on the chroma residual samples, wherein in the scaling process, the chroma residual samples are scaled based on at least one scaling factor before being used to reconstruct the current chroma video block, and wherein the at least one scaling factor is derived based on an averaged luma variable computed based on neighboring luma samples of a video unit of the video which is determined based on a luma sample corresponding to a top-left sample of the current chroma video block, and wherein the at least one scaling factor is derived by: checking an availability of each of one or more neighboring luma blocks of the video unit, wherein the each of the one or more neighboring luma blocks comprise at least one sample of the neighboring luma samples, determining, based on the availability of the each of the one or more neighboring luma blocks, whether to retrieve the neighboring luma samples of the video unit, deriving the at least one scaling factor based on the averaged luma variable computed using the neighboring luma samples by a rounding-based average operation in response to the neighboring luma samples being available, and deriving the at least one scaling factor as 1<<(bitDepth−1) in response to the neighboring luma samples being unavailable, where bitDepth is a bit depth of the video, and wherein a size information of a virtual pipeline data unit is further used to derive the at least one scaling factor.
 15. A non-transitory computer-readable storage medium storing instructions that cause a processor to: determine, during a conversion between a current chroma video block of a video and a bitstream of the video, that a scaling process is applied on chroma residual samples of the current chroma video block; and perform the conversion by applying the scaling process on the chroma residual samples, wherein in the scaling process, the chroma residual samples are scaled based on at least one scaling factor before being used to reconstruct the current chroma video block, and wherein the at least one scaling factor is derived based on an averaged luma variable computed based on neighboring luma samples of a video unit of the video which is determined based on a luma sample corresponding to a top-left sample of the current chroma video block, and wherein the at least one scaling factor is derived by: checking an availability of each of one or more neighboring luma blocks of the video unit, wherein the each of the one or more neighboring luma blocks comprise at least one sample of the neighboring luma samples, determining, based on the availability of the each of the one or more neighboring luma blocks, whether to retrieve the neighboring luma samples of the video unit, and deriving the at least one scaling factor based on the averaged luma variable computed using the neighboring luma samples by a rounding-based average operation in response to the neighboring luma samples being available, and deriving the at least one scaling factor as 1<<(bitDepth−1) in response to the neighboring luma samples being unavailable, where bitDepth is a bit depth of the video, and wherein a size information of a virtual pipeline data unit is further used to derive the at least one scaling factor.
 16. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by a video processing apparatus, wherein the method comprises: determining that a scaling process is applied on chroma residual samples of the current chroma video block; and generating the bitstream by applying the scaling process on the chroma residual samples, wherein in the scaling process, the chroma residual samples are scaled based on at least one scaling factor before being used to reconstruct the current chroma video block, and wherein the at least one scaling factor is derived based on an averaged luma variable computed based on neighboring luma samples of a video unit of the video which covers a luma sample corresponding to a top-left sample of the current chroma video block, and wherein the at least one scaling factor is derived by: checking an availability of each of one or more neighboring luma blocks of the video unit, wherein the each of the one or more neighboring luma blocks comprise at least one sample of the neighboring luma samples, determining, based on the availability of the each of the one or more neighboring luma blocks, whether to retrieve the neighboring luma samples of the video unit, and deriving the at least one scaling factor based on the averaged luma variable computed using the neighboring luma samples by a rounding-based average operation in response to the neighboring luma samples are available, and deriving the at least one scaling factor as 1<<(bitDepth−1) in response to the neighboring luma samples being unavailable, where bitDepth is a bit depth of the video, and wherein a size information of a virtual pipeline data unit is further used to derive the at least one scaling factor.
 17. The apparatus of claim 14, wherein the neighboring luma samples are located in predefined positions neighboring to the video unit, wherein the neighboring luma samples located in the predefined positions neighboring to the video unit include reconstructed luma samples adjacent to the video unit, wherein the reconstructed luma samples adjacent to the video unit include at least one of one or more left neighboring luma sample columns or one or more above neighboring luma sample rows of the video unit.
 18. The apparatus of claim 14, wherein a total number of the neighboring luma samples of the video unit is N, where N is an integer greater than 1, and a range of N depends on a size information of the video unit.
 19. The apparatus of claim 14, wherein the checking the availability of each of one or more neighboring luma blocks comprises: checking the availability of each of one or more neighboring luma blocks based on at least one of a width and a height of a video region of the video.
 20. The apparatus of claim 19, wherein, in response to a top-left sample of the one neighboring luma block being located outside the video region, the neighboring luma block is treated as unavailable. 